[mips] Attempting to use register $32 should be an error instead of an assertion.

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3201

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204932 91177308-0d34-0410-b5e6-96231b3b80d8