[llvm] [AMDGPU][InstCombine] Fold ballot intrinsic based on llvm.assume hints (PR #160670)

[Matt Arsenault via llvm-commits]

================
@@ -1341,6 +1342,73 @@ GCNTTIImpl::instCombineIntrinsic(InstCombiner &IC, IntrinsicInst &II) const {
       Call->takeName(&II);
       return IC.replaceInstUsesWith(II, Call);
     }
+
+    // Fold ballot intrinsic based on llvm.assume hint about the result.
+    //
+    // assume(ballot(x) == ballot(true)) -> x = true
+    // assume(ballot(x) == -1)           -> x = true
+    // assume(ballot(x) == 0)            -> x = false
+    //
+    // Skip if Arg is not an instruction (e.g., constant, argument).
+    if (!isa<Instruction>(Arg))
+      break;
+
+    // Skip if ballot width < wave size (e.g., ballot.i32 on wave64).
+    if (ST->isWave64() && II.getType()->getIntegerBitWidth() == 32)
----------------
arsenm wrote:

```suggestion
    if (WavefrontSize != II.getType()->getIntegerBitWidth())
```

https://github.com/llvm/llvm-project/pull/160670