[llvm] [SLP][NFCI] Address issues seen in downstream Coverity scan. (PR #93757)

[Tyler Lanphear via llvm-commits]

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@@ -11700,8 +11700,7 @@ class BoUpSLP::ShuffleInstructionBuilder final : public BaseShuffleAnalysis {
       R.eraseInstruction(EI);
     }
     if (NumParts == 1 || UniqueBases.size() == 1) {
-      VecBase = castToScalarTyElem(VecBase);
-      return VecBase;
+      return VecBase ? castToScalarTyElem(VecBase) : nullptr;
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tylanphear wrote:

Hm, maybe it makes sense to add an `assert` there? I'm not sure `nullptr` is an expected input for this function, but I'm admittedly not too familiar with the SLP Vectorizer.

https://github.com/llvm/llvm-project/pull/93757