[X86] getFauxShuffleMask - only handle VTRUNC nodes with matching src/dst sizes

Cleanup work for llvm#133947 - we need to handle VTRUNC nodes with large source vectors directly to allow us to widen the size of the shuffle combine

We currently discard these results in combineX86ShufflesRecursively anyhow as we don't allow inputs from getTargetShuffleInputs to be larger than the shuffle value type

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -6413,9 +6413,7 @@ static bool getFauxShuffleMask(SDValue N, const APInt &DemandedElts,
   case X86ISD::VTRUNC: {
     SDValue Src = N.getOperand(0);
     EVT SrcVT = Src.getValueType();
-    // Truncated source must be a simple vector.
-    if (!SrcVT.isSimple() || (SrcVT.getSizeInBits() % 128) != 0 ||
-        (SrcVT.getScalarSizeInBits() % 8) != 0)
+    if (SrcVT.getSizeInBits() != NumSizeInBits)
       return false;
     unsigned NumSrcElts = SrcVT.getVectorNumElements();
     unsigned NumBitsPerSrcElt = SrcVT.getScalarSizeInBits();