[SLP] put verifyFunction call behind EXPENSIVE_CHECKS

[spatel]

Summary

A severe compile-time slowdown from this call is noted in:
https://llvm.org/PR48689
My naive fix was to put it under LLVM_DEBUG ( 267ff79 ), but that's not limiting in the way we want.
IIUC, this is an acceptable quick fix (or we could just remove the call completely and rely on some later pass to discover potentially wrong IR?).
A bigger/better fix would be to improve/limit verifyFunction() as noted in:
https://llvm.org/PR47712

[nikic]

LGTM

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

for (const auto &Pair : DeletedInstructions) {
     }
     Pair.getFirst()->dropAllReferences();
   }
   for (const auto &Pair : DeletedInstructions) {
     assert(Pair.getFirst()->use_empty() &&
            "trying to erase instruction with users.");
     Pair.getFirst()->eraseFromParent();
   }
-  LLVM_DEBUG(verifyFunction(*F));
+#ifdef EXPENSIVE_CHECKS
+  // If we could guarantee that this call is not extremely slow, we could
+  // remove the ifdef limitation (see PR47712).
+  assert(!verifyFunction(*F, %dbgs()));
+#endif
 }
 
 void BoUpSLP::eraseInstructions(ArrayRef<Value *> AV) {
   for (auto *V : AV) {
     if (auto *I = dyn_cast<Instruction>(V))
       eraseInstruction(I, /*ReplaceOpsWithUndef=*/true);
   };
 }