[mips] Eliminate instances of "potentially uninitialised local variable" warnings, NFC

Summary:
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
warnings.

Reviewers: ariccio, vkalintiris, dsanders

Subscribers: dblaikie, dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D18087

llvm-svn: 263703