[X86] In lowerVectorShuffleAsBroadcast, make peeking through CONCAT_VECTORS work correctly if we already walked through a bitcast that changed the element size.

[craig.topper]

Summary

The CONCAT_VECTORS case was using the original mask element count to determine how to adjust the broadcast index. But if we looked through a bitcast the original mask size doesn't tell us anything about the concat_vectors.

This patch switchs to using the concat_vectors input element count directly instead.

This caused a crash while doing experiments with -mprefer-vector-width=256 with skylake-avx512 on some benchmarks. All the types present in the crash were 256 or 128 bits wide so I'm not sure why -mprefer-vector-width=256 was relevant.

I don't currently have a reduced test case, but I'll see what I can do.

[craig.topper]

Here's a test case that crashes with -mattr=avx2

define <8 x float> @foo(<4 x float> %x, <4 x float> %y, float %z) {
entry:
  %tmp = shufflevector <4 x float> %x, <4 x float> %y, <8 x i32> <i32 0, i32 1, i32 2, i32 3, i32 4, i32 5, i32 6, i32 7>
  %bc = bitcast <8 x float> %tmp to <4 x i64>
  %tmp1 = extractelement <4 x i64> %bc, i32 3
  %tmp2 = bitcast i64 %tmp1 to <2 x float>
  %tmp4 = extractelement <2 x float> %tmp2, i32 1
  %tmp5 = insertelement <8 x float> undef, float %tmp4, i32 4
  %tmp6 = insertelement <8 x float> %tmp5, float %z, i32 5
  ret <8 x float> %tmp6
}

[RKSimon]

LGTM - please can you add that test to the vector-shuffle-256-v8.ll file?

lib/Target/X86/X86ISelLowering.cpp

case ISD::BITCAST: {
                (BroadcastIdx % (NumSrcBits / NumEltBits)) == 0)
         BroadcastIdx /= (NumSrcBits / NumEltBits);
       else
         break;
       V = VSrc;
       continue;
     }
     case ISD::CONCAT_VECTORS: {
-      int OperandSize = Mask.size() / V.getNumOperands();
+      int OperandSize =
+          V.getOperand(0).getSimpleValueType().getVectorNumElements();
       V = V.getOperand(BroadcastIdx / OperandSize);
       BroadcastIdx %= OperandSize;
       continue;
     }
     case ISD::INSERT_SUBVECTOR: {
       SDValue VOuter = V.getOperand(0), VInner = V.getOperand(1);
       auto ConstantIdx = dyn_cast<ConstantSDNode>(V.getOperand(2));
       if (!ConstantIdx)