Add basic functionality for assignment of ints.
This creates a lot of core infrastructure in which to add, with little
effort, quite a bit more to mips fast-isel

Test Plan: simplestore.ll

Reviewers: dsanders

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D3527

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207790 91177308-0d34-0410-b5e6-96231b3b80d8