nir: Make nir_opt_remove_phis see through moves.

nir: Make nir_opt_remove_phis see through moves.

I found a shader in Tales of Maj'Eyal that contains:

        if ssa_21 {
                block block_1:
                /* preds: block_0 */
                ...instructions that prevent the select peephole...
                vec1 32 ssa_23 = imov ssa_4
                vec1 32 ssa_24 = imov ssa_4.y
                vec1 32 ssa_25 = imov ssa_4.z
                /* succs: block_3 */
        } else {
                block block_2:
                /* preds: block_0 */
                vec1 32 ssa_26 = imov ssa_4
                vec1 32 ssa_27 = imov ssa_4.y
                vec1 32 ssa_28 = imov ssa_4.z
                /* succs: block_3 */
        }
        block block_3:
        /* preds: block_1 block_2 */
        vec1 32 ssa_29 = phi block_1: ssa_23, block_2: ssa_26
        vec1 32 ssa_30 = phi block_1: ssa_24, block_2: ssa_27
        vec1 32 ssa_31 = phi block_1: ssa_25, block_2: ssa_28

Here, copy propagation will bail because phis cannot perform swizzles,
and CSE won't do anything because there is no dominance relationship
between the imovs.  By making nir_opt_remove_phis handle identical moves,
we can eliminate the phis and rewrite everything to use ssa_4 directly,
so all the moves become dead and get eliminated.

I don't think we need to check "exact" - just the alu sources.
Presumably phi sources should match in their exactness.

On Broadwell:

total instructions in shared programs: 11639872 -> 11638535 (-0.01%)
instructions in affected programs: 134222 -> 132885 (-1.00%)
helped: 338
HURT: 0

v2: Fix return value to be NULL, not false (caught by Iago).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>