R600/SI: Merge offset0 and offset1 fields for single address DS instructions v2

Also remove unused data fields from the DS_Load_Helper class.

v2:
  - Merge fields for DS_WRITE
R600/SI: Don't display the GDS bit.

It isn't actually used now, and probably never will be, plus it makes
tests less annoying. I also think SC prints GDS instructions as a
separate instruction name.
R600/SI: Fix test checking wrong instruction operand.

The source and destination happen to be the same register.
R600/SI: Match i16 immediate offset of LDS instructions.
R600/SI: Add support for 64-bit LDS loads.

v2:
  -Use correct opcode for DS_READ_64
R600/SI: Add support for 64-bit LDS writes
R600/SI: Add unused LDS 2 form instructions.