[mips] Correct the definitions of the unaligned word memory operation instructions

These instructions lacked the correct predicates, were not marked
as loads and stores and lacked the proper instruction mapping information.

In the case of microMIPS sw(l|r)e (EVA) these instructions were using the load
EVA description.

Reviewers: abeserminji, smaksimovic, atanasyan

Differential Revision: https://reviews.llvm.org/D45626

llvm-svn: 330326