[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