Rename loop unrolling and loop vectorizer metadata to have a common prefix.

[LLVM part]

These patches rename the loop unrolling and loop vectorizer metadata
such that they have a common 'llvm.loop.' prefix.  Metadata name
changes:

llvm.vectorizer.* => llvm.loop.vectorizer.*
llvm.loopunroll.* => llvm.loop.unroll.*

This was a suggestion from an earlier review
(http://reviews.llvm.org/D4090) which added the loop unrolling
metadata. 

Patch by Mark Heffernan.

llvm-svn: 211710

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1062,7 +1062,8 @@ std::error_code BitcodeReader::ParseMetadata() {
       break;
     }
     case bitc::METADATA_STRING: {
-      SmallString<8> String(Record.begin(), Record.end());
+      std::string String(Record.begin(), Record.end());
+      llvm::UpgradeMDStringConstant(String);
       Value *V = MDString::get(Context, String);
       MDValueList.AssignValue(V, NextMDValueNo++);
       break;