[X86] combineConcatVectorOps - fold(undef,zero) -> zero

Concat any mixture of zero/undefs to zero (we've already handled all undefs at this point).

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -57873,7 +57873,7 @@ static SDValue combineConcatVectorOps(const SDLoc &DL, MVT VT,
     return DAG.getUNDEF(VT);
 
   if (llvm::all_of(Ops, [](SDValue Op) {
-        return ISD::isBuildVectorAllZeros(Op.getNode());
+        return Op.isUndef() || ISD::isBuildVectorAllZeros(Op.getNode());
       }))
     return getZeroVector(VT, Subtarget, DAG, DL);
 

llvm/test/CodeGen/X86/vector-shuffle-combining-avx512bwvl.ll

@@ -191,11 +191,10 @@ define void @PR46178(ptr %0) {
 ; X86-NEXT:    vmovdqu (%eax), %ymm1
 ; X86-NEXT:    vpmovqw %ymm0, %xmm0
 ; X86-NEXT:    vpmovqw %ymm1, %xmm1
-; X86-NEXT:    vpsllw $8, %xmm0, %xmm0
-; X86-NEXT:    vpsraw $8, %xmm0, %xmm0
-; X86-NEXT:    vpsllw $8, %xmm1, %xmm1
-; X86-NEXT:    vpsraw $8, %xmm1, %xmm1
-; X86-NEXT:    vpunpcklqdq {{.*#+}} ymm0 = ymm0[0],ymm1[0],ymm0[2],ymm1[2]
+; X86-NEXT:    vinserti128 $1, %xmm1, %ymm0, %ymm0
+; X86-NEXT:    vpsllw $8, %ymm0, %ymm0
+; X86-NEXT:    vpsraw $8, %ymm0, %ymm0
+; X86-NEXT:    vpermq {{.*#+}} ymm0 = ymm0[0,2,1,1]
 ; X86-NEXT:    vmovdqu %ymm0, (%eax)
 ; X86-NEXT:    vzeroupper
 ; X86-NEXT:    retl