pan/midgard: Add post-RA move elimination

pan/midgard: Add post-RA move elimination

Think of this pass as register coalescing part 2. After RA runs, but
before scheduling, we scan for code of the form:

   mov rN, rN

and delete the move, since it's totally redundant. This pass helps
already, but it'd of course be much more effective paired with
register coalescing to encourage moves in general to end up in this
form. Nevertheless, even by itself:

total instructions in shared programs: 3665 -> 3613 (-1.42%)
instructions in affected programs: 2046 -> 1994 (-2.54%)
helped: 52
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.19% max: 25.00% x̄: 8.02% x̃: 4.00%
95% mean confidence interval for instructions value: -1.00 -1.00
95% mean confidence interval for instructions %-change: -10.26% -5.79%
Instructions are helped.

total bundles in shared programs: 2256 -> 2213 (-1.91%)
bundles in affected programs: 1154 -> 1111 (-3.73%)
helped: 43
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.33% max: 25.00% x̄: 9.10% x̃: 5.56%
95% mean confidence interval for bundles value: -1.00 -1.00
95% mean confidence interval for bundles %-change: -11.60% -6.60%
Bundles are helped.

total quadwords in shared programs: 3689 -> 3642 (-1.27%)
quadwords in affected programs: 2025 -> 1978 (-2.32%)
helped: 47
HURT: 0
helped stats (abs) min: 1 max: 1 x̄: 1.00 x̃: 1
helped stats (rel) min: 0.19% max: 25.00% x̄: 7.86% x̃: 3.85%
95% mean confidence interval for quadwords value: -1.00 -1.00
95% mean confidence interval for quadwords %-change: -10.30% -5.42%
Quadwords are helped.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>