[mips] Correct testcase for .set at=$reg and emit the new warnings for numeric registers too.

Summary:
Remove the XFAIL added in my previous commit and correct the test such that
it correctly tests the expansion of the assembler temporary.

Also added a test to check that $at is always $1 when written by the
user.

Corrected the new assembler temporary warnings so that they are emitted for
numeric registers too.

Differential Revision: http://llvm-reviews.chandlerc.com/D3169

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204711 91177308-0d34-0410-b5e6-96231b3b80d8