[llvm] [InstCombine] Extend bitmask mul combine to handle independent operands (PR #142503)

[Nikita Popov via llvm-commits]

================
@@ -3659,6 +3665,64 @@ static std::optional<DecomposedBitMaskMul> matchBitmaskMul(Value *V) {
   return std::nullopt;
 }
 
+/// (A & N) * C + (A & M) * C -> (A & (N + M)) & C
+/// This also accepts the equivalent select form of (A & N) * C
+/// expressions i.e. !(A & N) ? 0 : N * C)
+static Value *foldBitmaskMul(Value *Op0, Value *Op1,
+                             InstCombiner::BuilderTy &Builder) {
+  auto Decomp1 = matchBitmaskMul(Op1);
+
+  if (Decomp1) {
+    auto Decomp0 = matchBitmaskMul(Op0);
+
+    if (Decomp0) {
+      // If we have independent operands in the BitmaskMul chain, then just
+      // reassociate to encourage combining in future iterations.
+
+      if (Decomp0->isCombineableWith(*Decomp1)) {
+        auto NewAnd = Builder.CreateAnd(
----------------
nikic wrote:

```suggestion
        Value *NewAnd = Builder.CreateAnd(
```
(or at least `auto *`.)

https://github.com/llvm/llvm-project/pull/142503