[llvm] [PatternMatch] Add a matching helper `m_ElementWiseBitCast`. NFC. (PR #80764)

[Yingwei Zheng via llvm-commits]

================
@@ -182,9 +182,15 @@ Instruction *InstCombinerImpl::commonCastTransforms(CastInst &CI) {
     if (!Cmp || Cmp->getOperand(0)->getType() != Sel->getType() ||
         (CI.getOpcode() == Instruction::Trunc &&
          shouldChangeType(CI.getSrcTy(), CI.getType()))) {
-      if (Instruction *NV = FoldOpIntoSelect(CI, Sel)) {
-        replaceAllDbgUsesWith(*Sel, *NV, CI, DT);
-        return NV;
+
+      // If it's a bitcast involving vectors, make sure it has the same number
+      // of elements on both sides.
+      if (CI.getOpcode() != Instruction::BitCast ||
+          match(&CI, m_ElementWiseBitCast(m_Value()))) {
+        if (Instruction *NV = FoldOpIntoSelect(CI, Sel)) {
+          replaceAllDbgUsesWith(*Sel, *NV, CI, DT);
+          return NV;
----------------
dtcxzyw wrote:

I removed the check in `InstructionCombing.cpp`:
```
 // If it's a bitcast involving vectors, make sure it has the same number of
 // elements on both sides.
 if (auto *BC = dyn_cast<BitCastInst>(&Op)) {
   VectorType *DestTy = dyn_cast<VectorType>(BC->getDestTy());
   VectorType *SrcTy = dyn_cast<VectorType>(BC->getSrcTy());

   // Verify that either both or neither are vectors.
   if ((SrcTy == nullptr) != (DestTy == nullptr))
     return nullptr;

   // If vectors, verify that they have the same number of elements.
   if (SrcTy && SrcTy->getElementCount() != DestTy->getElementCount())
     return nullptr;
 }
```

https://github.com/llvm/llvm-project/pull/80764