[clang] [llvm] Match bitsin(typeof(x)) - popcnt(x) to s_bcnt0_i32 on AMDGPU (PR #164847)

[Jay Foad via cfe-commits]

================
@@ -465,17 +463,16 @@ define amdgpu_ps i32 @bcnt032(i32 inreg %val0) {
 define amdgpu_ps i32 @bcnt064(i64 inreg %val0) {
 ; CHECK-LABEL: bcnt064:
 ; CHECK:       ; %bb.0:
-; CHECK-NEXT:    s_bcnt1_i32_b64 s0, s[0:1]
-; CHECK-NEXT:    s_sub_u32 s0, 64, s0
-; CHECK-NEXT:    s_subb_u32 s1, 0, 0
-; CHECK-NEXT:    s_cmp_lg_u64 s[0:1], 0
-; CHECK-NEXT:    ;;#ASMSTART
-; CHECK-NEXT:    ; use s[0:1]
-; CHECK-NEXT:    ;;#ASMEND
-; CHECK-NEXT:    s_cselect_b64 s[0:1], -1, 0
-; CHECK-NEXT:    v_cndmask_b32_e64 v0, 0, 1, s[0:1]
-; CHECK-NEXT:    v_readfirstlane_b32 s0, v0
-; CHECK-NEXT:    ; return to shader part epilog
+; CHECK-NEXT: s_bcnt0_i32_b64 s0, s[0:1]
+; CHECK-NEXT: s_mov_b32 s1, 0
+; CHECK-NEXT: s_cmp_lg_u64 s[0:1], 0
----------------
jayfoad wrote:

As a follow up, it would be good if we could somehow generate a 32-bit compare instead here:
```suggestion
; CHECK-NEXT: s_cmp_lg_u32 s0, 0
```

https://github.com/llvm/llvm-project/pull/164847