[mips][atomics] Fix atomic instruction descriptions and uses.

PR27458 highlights that the MIPS backend does not have well formed
MIR for atomic operations (among other errors).

This patch adds expands and corrects the LL/SC descriptions and uses
for MIPS(64).

Reviewers: dsanders, vkalintiris

Differential Review: http://reviews.llvm.org/D19719

llvm-svn: 272655