[DAGCombiner] Don't fold zext_vector_inreg/sext_vector_inreg(undef) to undef. Fold to 0.

zext_vector_inreg needs to produces 0s in the extended bits and
sext_vector_inreg needs to produce upper bits that are all the
same. So we should fold them to a 0 vector instead of undef.

Fixes PR46585.

Author: topperc

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -11086,8 +11086,9 @@ SDValue DAGCombiner::visitSIGN_EXTEND_VECTOR_INREG(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
+  // sext_vector_inreg(undef) = 0 because the top bit will all be the same.
   if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+    return DAG.getConstant(0, SDLoc(N), VT);
 
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;
@@ -11102,8 +11103,9 @@ SDValue DAGCombiner::visitZERO_EXTEND_VECTOR_INREG(SDNode *N) {
   SDValue N0 = N->getOperand(0);
   EVT VT = N->getValueType(0);
 
+  // zext_vector_inreg(undef) = 0 because the top bits will be zero.
   if (N0.isUndef())
-    return DAG.getUNDEF(VT);
+    return DAG.getConstant(0, SDLoc(N), VT);
 
   if (SDValue Res = tryToFoldExtendOfConstant(N, TLI, DAG, LegalTypes))
     return Res;

llvm/test/CodeGen/X86/pr46585.ll

@@ -7,7 +7,7 @@
 define void @spam() local_unnamed_addr {
 ; CHECK-LABEL: spam:
 ; CHECK:       ## %bb.0: ## %bb
-; CHECK-NEXT:    pmovmskb %xmm0, %eax
+; CHECK-NEXT:    xorl %eax, %eax
 ; CHECK-NEXT:    testb %al, %al
 ; CHECK-NEXT:    je LBB0_2
 ; CHECK-NEXT:  ## %bb.1: ## %bb9