nir: Copy propagation between blocks

nir: Copy propagation between blocks

Extend the pass to propagate the copies information along the control
flow graph.  It performs two walks, first it collects the vars
that were written inside each node. Then it walks applying the copy
propagation using a list of copies previously available.  At each node
the list is invalidated according to results from the first walk.

This approach is simpler than a full data-flow analysis, but covers
various cases.  If derefs are used for operating on more memory
resources (e.g. SSBOs), the difference from a regular pass is expected
to be more visible -- as the SSA copy propagation pass won't apply to
those.

A full data-flow analysis would handle more scenarios: conditional
breaks in the control flow and merge equivalent effects from multiple
branches (e.g. using a phi node to merge the source for writes to the
same deref).  However, as previous commentary in the code stated, its
complexity 'rapidly get out of hand'.  The current patch is a good
intermediate step towards more complex analysis.

The 'copies' linked list was modified to use util_dynarray to make it
more convenient to clone it (to handle ifs/loops).

Annotated shader-db results for Skylake:

    total instructions in shared programs: 15105796 -> 15105451 (<.01%)
    instructions in affected programs: 152293 -> 151948 (-0.23%)
    helped: 96
    HURT: 17

        All the HURTs and many HELPs are one instruction.  Looking
        at pass by pass outputs, the copy prop kicks in removing a
        bunch of loads correctly, which ends up altering what other
        other optimizations kick.  In those cases the copies would be
        propagated after lowering to SSA.

        In few HELPs we are actually helping doing more than was
        possible previously, e.g. consolidating load_uniforms from
        different blocks.  Most of those are from
        shaders/dolphin/ubershaders/.

    total cycles in shared programs: 566048861 -> 565954876 (-0.02%)
    cycles in affected programs: 151461830 -> 151367845 (-0.06%)
    helped: 2933
    HURT: 2950

        A lot of noise on both sides.

    total loops in shared programs: 4603 -> 4603 (0.00%)
    loops in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total spills in shared programs: 11085 -> 11073 (-0.11%)
    spills in affected programs: 23 -> 11 (-52.17%)
    helped: 1
    HURT: 0

        The shaders/dolphin/ubershaders/12.shader_test was able to
        pull a couple of loads from inside if statements and reuse
        them.

    total fills in shared programs: 23143 -> 23089 (-0.23%)
    fills in affected programs: 2718 -> 2664 (-1.99%)
    helped: 27
    HURT: 0

        All from shaders/dolphin/ubershaders/.

    LOST:   0
    GAINED: 0

The other generations follow the same overall shape.  The spills and
fills HURTs are all from the same game.

shader-db results for Broadwell.

    total instructions in shared programs: 15402037 -> 15401841 (<.01%)
    instructions in affected programs: 144386 -> 144190 (-0.14%)
    helped: 86
    HURT: 9

    total cycles in shared programs: 600912755 -> 600902486 (<.01%)
    cycles in affected programs: 185662820 -> 185652551 (<.01%)
    helped: 2598
    HURT: 3053

    total loops in shared programs: 4579 -> 4579 (0.00%)
    loops in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total spills in shared programs: 80929 -> 80924 (<.01%)
    spills in affected programs: 720 -> 715 (-0.69%)
    helped: 1
    HURT: 5

    total fills in shared programs: 93057 -> 93013 (-0.05%)
    fills in affected programs: 3398 -> 3354 (-1.29%)
    helped: 27
    HURT: 5

    LOST:   0
    GAINED: 2

shader-db results for Haswell:

    total instructions in shared programs: 9231975 -> 9230357 (-0.02%)
    instructions in affected programs: 44992 -> 43374 (-3.60%)
    helped: 27
    HURT: 69

    total cycles in shared programs: 87760587 -> 87727502 (-0.04%)
    cycles in affected programs: 7720673 -> 7687588 (-0.43%)
    helped: 1609
    HURT: 1416

    total loops in shared programs: 1830 -> 1830 (0.00%)
    loops in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total spills in shared programs: 1988 -> 1692 (-14.89%)
    spills in affected programs: 296 -> 0
    helped: 1
    HURT: 0

    total fills in shared programs: 2103 -> 1668 (-20.68%)
    fills in affected programs: 438 -> 3 (-99.32%)
    helped: 4
    HURT: 0

    LOST:   0
    GAINED: 1

v2: Remove the DISABLE prefix from tests we now pass.

v3: Add comments about missing write_mask handling. (Caio)
    Add unreachable when switching on cf_node type. (Jason)
    Properly merge the component information in written map
    instead of replacing. (Jason)
    Explain how removal from written arrays works. (Jason)
    Use mode directly from deref instead of getting the var. (Jason)

v4: Register the local written mode for calls. (Jason)
    Prefer cf_node instead of node. (Jason)
    Clarify that remove inside iteration only works in backward
    iterations. (Jason)

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>