[mips] Improve support for the .set at/noat assembler directives.

Summary:
Made the following changes:
  Added calls to emitDirectiveSetNoAt() and emitDirectiveSetAt().
  Added special emit function for .set at=$reg, emitDirectiveSetAtWithArg(unsigned RegNo).
  Improved parsing error checks for .set at.
  Refactored parser code for .set at.
  Improved testing of both directives.
  Improved code readability and comments.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7176

llvm-svn: 229097