[SLP] limit verifyFunction to debug build (PR48689)

As noted in PR48689, the verifier may have some kind
of exponential behavior that should be addressed
separately. For now, only run it in debug mode to
prevent problems for release+asserts.
That limit is what we had before D80401, and I'm
not sure if there was a reason to change it in that
patch.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -2499,7 +2499,7 @@ BoUpSLP::~BoUpSLP() {
            "trying to erase instruction with users.");
     Pair.getFirst()->eraseFromParent();
   }
-  assert(!verifyFunction(*F, &dbgs()));
+  LLVM_DEBUG(verifyFunction(*F));
 }
 
 void BoUpSLP::eraseInstructions(ArrayRef<Value *> AV) {