Mips specific inline assembler constraint 'R'

'R' An address that can be sued in a non-macro load or store.

Including missing positive test case and fixed typo for r176453.

Thanks to Richard Smith for catching this!

Jack



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176506 91177308-0d34-0410-b5e6-96231b3b80d8