[InstCombine] remove overzealous assert for shuffles (PR41419)

As the TODO indicates, instsimplify could be improved.

Should fix:
https://bugs.llvm.org/show_bug.cgi?id=41419

llvm-svn: 357910

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -1353,8 +1353,8 @@ static Instruction *foldSelectShuffle(ShuffleVectorInst &Shuf,
   // Canonicalize to choose from operand 0 first.
   unsigned NumElts = Shuf.getType()->getVectorNumElements();
   if (Shuf.getMaskValue(0) >= (int)NumElts) {
-    assert(!isa<UndefValue>(Shuf.getOperand(1)) &&
-           "Not expecting undef shuffle operand with select mask");
+    // TODO: Can we assert that both operands of a shuffle-select are not undef
+    // (otherwise, it would have been folded by instsimplify?
     Shuf.commute();
     return &Shuf;
   }

llvm/test/Transforms/InstCombine/shuffle_select.ll

@@ -1454,3 +1454,13 @@ define <4 x i8> @or_add_2_vars(<4 x i8> %v, <4 x i8> %v1) {
   ret <4 x i8> %t3
 }
 
+; The undef operand is used to simplify the shuffle mask, but don't assert that too soon.
+
+define <4 x i32> @PR41419(<4 x i32> %v) {
+; CHECK-LABEL: @PR41419(
+; CHECK-NEXT:    ret <4 x i32> [[V:%.*]]
+;
+  %s = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 4, i32 5, i32 2, i32 7>
+  ret <4 x i32> %s
+}
+