R600: Add 64-bit float load/store support

* Added R600_Reg64 class
* Added T#Index#.XY registers definition
* Added v2i32 register reads from parameter and global space
* Added f32 and i32 elements extraction from v2f32 and v2i32
* Added v2i32 -> v2f32 conversions

Tom Stellard:
  - Mark vec2 operations as expand.  The addition of a vec2 register
    class made them all legal.

Patch by: Dmitry Cherkassov

Signed-off-by: Dmitry Cherkassov <dcherkassov@gmail.com>
llvm-svn: 187582