i965/fs: Do live variables dataflow analysis on a per-channel level.

i965/fs: Do live variables dataflow analysis on a per-channel level.

This significantly improves our handling of VGRFs of size > 1.

Previously, we only marked VGRFs as def'd if the whole register was
written by a single instruction.  Large VGRFs which were written
piecemeal would not be considered def'd at all, even if they were
ultimately completely written.

Without being def'd, these were then marked "live in" to the basic
block, often extending the range to preceding blocks and sometimes
even the start of the program.

The new per-component tracking gives more accurate live intervals,
which makes register coalescing more effective.

In the future, this should help with texturing from GRFs on Gen7+.
A sampler message might be represented by a 2-register VGRF which
holds the texture coordinates.  If those are incoming varyings,
they'll be produced by two PLN instructions, which are piecemeal writes.

No reduction in shader-db instruction counts.  However, code which
prints the live interval ranges does show that some VGRFs now have
smaller (and more correct) live intervals.

v2: Rebase on current send-from-GRF code requiring adding extra use[]s.
v3: Rebase on live intervals fix to include defs in the end of the
    interval.
v4 (Kenneth Graunke): Rebase; split off a few preparatory patches;
    add lots of comments; minor style changes; rewrite commit message.
v5 (Eric Anholt): whitespace nit.

Written-by: Eric Anholt <eric@anholt.net> [v1-3]
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org> [v4]
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net> (v4)