[llvm] [InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (PR #148593)

[Luke Lau via llvm-commits]

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@@ -708,11 +708,17 @@ static Instruction *shrinkSplatShuffle(TruncInst &Trunc,
   auto *Shuf = dyn_cast<ShuffleVectorInst>(Trunc.getOperand(0));
   if (Shuf && Shuf->hasOneUse() && match(Shuf->getOperand(1), m_Undef()) &&
       all_equal(Shuf->getShuffleMask()) &&
-      Shuf->getType() == Shuf->getOperand(0)->getType()) {
+      ElementCount::isKnownGE(Shuf->getType()->getElementCount(),
+                              cast<VectorType>(Shuf->getOperand(0)->getType())
+                                  ->getElementCount())) {
     // trunc (shuf X, Undef, SplatMask) --> shuf (trunc X), Poison, SplatMask
     // trunc (shuf X, Poison, SplatMask) --> shuf (trunc X), Poison, SplatMask
-    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), Trunc.getType());
-    return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
+    auto *const NewTruncTy = Shuf->getOperand(0)->getType()->getWithNewType(
----------------
lukel97 wrote:

Style nit, avoid using auto when it doesn't make the type clearer https://llvm.org/docs/CodingStandards.html#id29

```suggestion
    Type *NewTruncTy = Shuf->getOperand(0)->getType()->getWithNewType(
```

https://github.com/llvm/llvm-project/pull/148593