[X86] visitSelect - widen select(cond,extract_subvector(shuffle(vec0)),vec1) if it will create a mask instruction

This patch extends the existing fold "select(mask, extract_subvector(shuffle(x)), zero) --> extract_subvector(select(insert_subvector(mask), shuffle(x), zero))", to also handle the non-zero case.

I've put in a restriction for VPERMV3 3 vector operands shuffles to only work with the zero select as in most circumstances we are not selecting with either of the source vectors (the only case the mask instructions match).

We should be able to generalise this in the future to work with other maskable instructions, but this is a good initial improvement.

Fixes #113400

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -46835,9 +46835,9 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
     return DAG.getNode(N->getOpcode(), DL, VT, Cond, LHS, RHS);
   }
 
-  // AVX512 - Extend select with zero to merge with target shuffle.
-  // select(mask, extract_subvector(shuffle(x)), zero) -->
-  // extract_subvector(select(insert_subvector(mask), shuffle(x), zero))
+  // AVX512 - Extend select to merge with target shuffle.
+  // select(mask, extract_subvector(shuffle(x)), y) -->
+  // extract_subvector(select(widen(mask), shuffle(x), widen(y)))
   // TODO - support non target shuffles as well with canCombineAsMaskOperation.
   if (Subtarget.hasAVX512() && CondVT.isVector() &&
       CondVT.getVectorElementType() == MVT::i1) {
@@ -46847,7 +46847,8 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
              isNullConstant(Op.getOperand(1)) &&
              TLI.isTypeLegal(Op.getOperand(0).getValueType()) &&
              Op.hasOneUse() && Op.getOperand(0).hasOneUse() &&
-             ISD::isBuildVectorAllZeros(Alt.getNode());
+             (Op.getOperand(0).getOpcode() != X86ISD::VPERMV3 ||
+              ISD::isBuildVectorAllZeros(Alt.getNode()));
     };
 
     bool SelectableLHS = SelectableOp(LHS, RHS);