[mips] Try to fix the test/ExecutionEngine tests on a MIPS host.

Fix a dangerous default case that caused MipsCodeEmitter to discard pseudo
instructions it didn't recognize. It will now call llvm_unreachable() for
unrecognized pseudo's and explicitly handles PseudoReturn, PseudoReturn64,
PseudoIndirectBranch, PseudoIndirectBranch64, CFI_INSTRUCTION, IMPLICIT_DEF,
and KILL.

There may be other pseudos that need handling but this was enough for the
ExecutionEngine tests to pass on my test system.