[mips] [IAS] Improve warning for using AT with .set noat.

Summary:
Changed the warning message to show the current value of $at, similar to what clang does for typedef's, and renamed warnIfAssemblerTemporary to a more descriptive name.

I also changed the type of variables which store registers from int to unsigned, updated the relevant test and tried to make the related comments clearer.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8479

llvm-svn: 235881