[Mesa-dev] R600: Various fixes for R600 and SI

[Aaron Watry]

First patch fixes load/store for v2i32 on R600. Without this, the
other two will cause make check failures.  I've verified the changes
using a Radeon 5400 (Cedar).  Note that the previous custom
lowering of v2i32 store was causing silent data corruption.

The other two patches expand add/sub on SI for both v2i32 and v4i32
types. There's lit tests for v2i32 that have been added.