Add numeric extend, trunctate to mips fast-isel

Summary:
 Add numeric extend, trunctate to mips fast-isel

 Reactivates D4827



Test Plan:
fpext.ll
loadstoreconv.ll

Reviewers: dsanders

Subscribers: mcrosier

Differential Revision: http://reviews.llvm.org/D5251

llvm-svn: 218681