[mips] Remove the last usage of parseRegister from MipsAsmParser.

Summary:
Added negative test case so that we can be sure we handle erroneous situations
while parsing the .cpsetup directive.

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3681

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211160 91177308-0d34-0410-b5e6-96231b3b80d8