[SLP][NFC] Add debug logs for entry.

Tell the users they are specifying something without vector register.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D117980

Author: eopXD

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -8079,8 +8079,11 @@ bool SLPVectorizerPass::runImpl(Function &F, ScalarEvolution *SE_,
 
   // If the target claims to have no vector registers don't attempt
   // vectorization.
-  if (!TTI->getNumberOfRegisters(TTI->getRegisterClassForType(true)))
+  if (!TTI->getNumberOfRegisters(TTI->getRegisterClassForType(true))) {
+    LLVM_DEBUG(
+        dbgs() << "SLP: Didn't find any vector registers for target, abort.\n");
     return false;
+  }
 
   // Don't vectorize when the attribute NoImplicitFloat is used.
   if (F.hasFnAttribute(Attribute::NoImplicitFloat))