[llvm] [GISel][CombinerHelper] Combine op(trunc(x), trunc(y)) -> trunc(op(x, y)) (PR #89023)

[Matt Arsenault via llvm-commits]

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@@ -290,9 +288,9 @@ define amdgpu_ps i16 @s_uaddsat_v2i8(i16 inreg %lhs.arg, i16 inreg %rhs.arg) {
 ; GFX6-NEXT:    s_min_u32 s2, s3, s2
 ; GFX6-NEXT:    s_add_i32 s1, s1, s2
 ; GFX6-NEXT:    s_lshr_b32 s1, s1, 24
-; GFX6-NEXT:    s_lshr_b32 s0, s0, 24
-; GFX6-NEXT:    s_lshl_b32 s1, s1, 8
-; GFX6-NEXT:    s_or_b32 s0, s0, s1
+; GFX6-NEXT:    v_mov_b32_e32 v0, s0
+; GFX6-NEXT:    v_alignbit_b32 v0, s1, v0, 24
+; GFX6-NEXT:    v_readfirstlane_b32 s0, v0
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arsenm wrote:

This is worse, but not the fault of this patch 

https://github.com/llvm/llvm-project/pull/89023