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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218681 91177308-0d34-0410-b5e6-96231b3b80d8