[SLP] Check invariant that all instructions in bundle are in same block [NFC]

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -2517,6 +2517,9 @@ private:
                  "only bundle is marked scheduled");
         }
       }
+
+      assert(Inst->getParent() == FirstInBundle->Inst->getParent() &&
+             "all bundle members must be in same basic block");
     }
 
     /// Returns true if the dependency information has been calculated.