[InstCombine] remove shuffle mask canonicalization that creates undef elements

This is NFC-intended because SimplifyDemandedVectorElts() does the same
transform later. As discussed in D70641, we may want to change that
behavior, so we need to isolate where it happens.

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -1886,24 +1886,18 @@ Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
           LHS, RHS, SVI.getMask(), SVI.getType(), SQ.getWithInstruction(&SVI)))
     return replaceInstUsesWith(SVI, V);
 
-  // Canonicalize shuffle(x    ,x,mask) -> shuffle(x, undef,mask')
-  // Canonicalize shuffle(undef,x,mask) -> shuffle(x, undef,mask').
+  // shuffle x, x, mask --> shuffle x, undef, mask'
   unsigned VWidth = SVI.getType()->getVectorNumElements();
   unsigned LHSWidth = LHS->getType()->getVectorNumElements();
   SmallVector<int, 16> Mask = SVI.getShuffleMask();
   Type *Int32Ty = Type::getInt32Ty(SVI.getContext());
-  if (LHS == RHS || isa<UndefValue>(LHS)) {
+  if (LHS == RHS) {
     assert(!isa<UndefValue>(RHS) && "Shuffle with 2 undef ops not simplified?");
     // Remap any references to RHS to use LHS.
     SmallVector<Constant*, 16> Elts;
     for (unsigned i = 0; i != VWidth; ++i) {
-      if (Mask[i] < 0) {
-        Elts.push_back(UndefValue::get(Int32Ty));
-        continue;
-      }
-
-      // Change select of undef to undef mask element or force to LHS.
-      if (Mask[i] < (int)LHSWidth && isa<UndefValue>(LHS))
+      // Propagate undef elements or force mask to LHS.
+      if (Mask[i] < 0)
         Elts.push_back(UndefValue::get(Int32Ty));
       else
         Elts.push_back(ConstantInt::get(Int32Ty, Mask[i] % LHSWidth));
@@ -1914,6 +1908,12 @@ Instruction *InstCombiner::visitShuffleVectorInst(ShuffleVectorInst &SVI) {
     return &SVI;
   }
 
+  // shuffle undef, x, mask --> shuffle x, undef, mask'
+  if (isa<UndefValue>(LHS)) {
+    SVI.commute();
+    return &SVI;
+  }
+
   if (Instruction *I = canonicalizeInsertSplat(SVI, Builder))
     return I;