[llvm] InstCombine: fix transformation of bitwiseAnd to fabs (PR #71257)

[Yingwei Zheng via llvm-commits]

================
@@ -2546,6 +2546,32 @@ Instruction *InstCombinerImpl::visitBitCast(BitCastInst &CI) {
   if (DestTy == Src->getType())
     return replaceInstUsesWith(CI, Src);
 
+  // If we are clearing the sign bit of a floating-point value, convert this to
+  // fabs
+  //
+  // This is a generous interpretation for noimplicitfloat, this is not a true
+  // floating-point operation.
+  //
+  // Assumes any IEEE-represented type has the sign bit in the high bit.
+  if (!Builder.GetInsertBlock()->getParent()->hasFnAttribute(
+          Attribute::NoImplicitFloat) &&
+      DestTy->isFloatingPointTy() && DestTy->isIEEE()) {
+    Value *L, *R;
+    if (match(Src, m_And(m_Value(L), m_Value(R)))) {
+      Value *Cast;
+      if (match(L, m_BitCast(m_Value(Cast))) && match(R, m_MaxSignedValue())) {
----------------
dtcxzyw wrote:

```suggestion
    Value *Cast;
    if (match(Src, m_And(m_BitCast(m_Value(Cast)), m_MaxSignedValue()))) {
```


https://github.com/llvm/llvm-project/pull/71257