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

[eopXD]

Summary

Tell the users they are specifying something without vector register.

[eopXD]

I was trying out the SLP vectorizer and find that adding some hint will be helpful.

[ABataev]

LG

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

bool SLPVectorizerPass::runImpl(Function &F, ScalarEvolution *SE_,
   DL = &F.getParent()->getDataLayout();
 
   Stores.clear();
   GEPs.clear();
   bool Changed = false;
 
   // 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))
     return false;
 
   LLVM_DEBUG(dbgs() << "SLP: Analyzing blocks in " << F.getName() << ".\n");
 
   // Use the bottom up slp vectorizer to construct chains that start with