[mips] Update MipsAsmParser so that it's possible to handle immediates that start with the binary operator NOT (~).

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D4158

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211163 91177308-0d34-0410-b5e6-96231b3b80d8