[mips] [IAS] Refactor the function which checks for the availability of AT. NFC.

Summary:
Refactor MipsAsmParser::getATReg to return an internal register number instead of a register index.
Also change all the int's to unsigned, seeing as the current AT register index is stored as an unsigned in MipsAssemblerOptions.



Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8478

llvm-svn: 234996