Convert a few std::string with StringRef. NFC.

This is a micro optimization, but also makes the code a bit more flexible.

The MRIMembers variable is a short term hack. It is going away in the next
commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220334 91177308-0d34-0410-b5e6-96231b3b80d8