util/ra: Improve the performance of ra_simplify

util/ra: Improve the performance of ra_simplify

The most expensive part of register allocation is the ra_simplify step
which is a fixed-point algorithm with a worst-case complexity of O(n^2)
which adds the registers to a stack which we then use later to do the
actual allocation.  This commit uses bit sets and changes the core loop
of ra_simplify to first walk 32-node chunks and then walk each chunk.
This lets us skip whole 32-node chunks in one go based on bit operations
and compute the minimum q value potentially 32x as fast.  Of course, the
algorithm still has the same fundamental O(n^2) run-time but the
constant is now much lower.

In the nasty Aztec Ruins compute shader, this shaves a full four seconds
off the 30s compile time for a release build of mesa.  In a debug build
(needed for accurate stack traces), perf says that ra_select takes 20%
of runtime before this patch and only 5-6% of runtime after this patch.
It also makes shader-db runs faster.

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15311100 -> 15311100 (0.00%)
    instructions in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total cycles in shared programs: 355468050 -> 355468050 (0.00%)
    cycles in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    Total CPU time (seconds): 2602.37 -> 2524.31 (-3.00%)

Reviewed-by: Eric Anholt <eric@anholt.net>