Reland "[mips][mt][6/7] Add support for mftr, mttr instructions."

This adjusts the tests to hopfully pacify the
llvm-clang-x86_64-expensive-checks-win buildbot.

Unlike many other instructions, these instructions have aliases which
take coprocessor registers, gpr register, accumulator (and dsp accumulator)
registers, floating point registers, floating point control registers and
coprocessor 2 data and control operands.

For the moment, these aliases are treated as pseudo instructions which are
expanded into the underlying instruction. As a result, disassembling these
instructions shows the underlying instruction and not the alias.

Reviewers: slthakur, atanasyan

Differential Revision: https://reviews.llvm.org/D35253

llvm-svn: 318207