[Utils] Avoid a hash table lookup in salvageDI, NFC

According to the current coverage report salvageDebugInfo() is called
5.12 million times during testing and almost always returns early.

The early return depends on LocalAsMetadata::getIfExists returning null,
which involves a DenseMap lookup in an LLVMContextImpl. We can probably
speed this up by simply checking the IsUsedByMD bit in Value.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325738 91177308-0d34-0410-b5e6-96231b3b80d8

Author: vedantk

lib/Transforms/Utils/Local.cpp

@@ -1486,6 +1486,11 @@ void llvm::replaceDbgValueForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
 }
 
 void llvm::salvageDebugInfo(Instruction &I) {
+  // This function is hot. An early check to determine whether the instruction
+  // has any metadata to save allows it to return earlier on average.
+  if (!I.isUsedByMetadata())
+    return;
+
   SmallVector<DbgInfoIntrinsic *, 1> DbgUsers;
   findDbgUsers(DbgUsers, &I);
   if (DbgUsers.empty())