[SLP] Fix OOB GEP index access for a no-op GEP

Issue is covered by existing test
llvm/test/Transforms/SLPVectorizer/RISCV/phi-const.ll

See issue #75632 for ideas for how we could catch these more easily in
the future.

Author: rnk

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -13682,8 +13682,10 @@ void SLPVectorizerPass::collectSeedInstructions(BasicBlock *BB) {
     // constant index, or a pointer operand that doesn't point to a scalar
     // type.
     else if (auto *GEP = dyn_cast<GetElementPtrInst>(&I)) {
+      if (GEP->getNumIndices() != 1)
+        continue;
       Value *Idx = GEP->idx_begin()->get();
-      if (GEP->getNumIndices() > 1 || isa<Constant>(Idx))
+      if (isa<Constant>(Idx))
         continue;
       if (!isValidElementType(Idx->getType()))
         continue;