[X86] In lowerVSELECTtoVectorShuffle, don't map undef select condition to undef in shuffle mask.

Undef in select condition means we should pick the element from one side or the other. An undef in a shuffle mask means pick any element from either source or worse.

I suspect by the time we get here most of the undefs in a constant vector have been removed by other things, but doing this for safety.

llvm-svn: 325394

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -14745,9 +14745,12 @@ static SDValue lowerVSELECTtoVectorShuffle(SDValue Op,
   SmallVector<int, 32> Mask;
   for (int i = 0, Size = VT.getVectorNumElements(); i < Size; ++i) {
     SDValue CondElt = CondBV->getOperand(i);
-    Mask.push_back(
-        isa<ConstantSDNode>(CondElt) ? i + (isNullConstant(CondElt) ? Size : 0)
-                                     : -1);
+    int M = i;
+    // We can't map undef to undef here. They have different meanings. Treat
+    // as the same as zero.
+    if (CondElt.isUndef() || isNullConstant(CondElt))
+      M += Size;
+    Mask.push_back(M);
   }
   return DAG.getVectorShuffle(VT, dl, LHS, RHS, Mask);
 }