[mips] Eliminate instances of "potentially uninitialised local variable" warnings, NFC
This should eliminate all occurrences of this within LLVMMipsAsmParser.
This patch is in response to http://reviews.llvm.org/D17983. I was unable
to reproduce the warnings on my machine so please advise if this fixes the
Reviewers: ariccio, vkalintiris, dsanders
Subscribers: dblaikie, dsanders, llvm-commits
Differential Revision: http://reviews.llvm.org/D18087