Revert: [mips] Rewrite MipsAsmParser and MipsOperand.' due to buildbot errors in lld tests.

It's currently unable to parse 'sym + imm' without surrounding parenthesis.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205237 91177308-0d34-0410-b5e6-96231b3b80d8