[mips] Enable the mnemonic spell corrector

This implements suggesting alternative mnemonics when an invalid one is
specified. For example `addru $9, $6, 17767` leads to the following
error message:

error: unknown instruction, did you mean: add, addiu, addu, maddu?

Differential revision: https://reviews.llvm.org/D40646

llvm-svn: 342119