R600/SI: Fix incorrect encoding of DS_WRITE_B32 instructions

The SIInsertWaits pass was overwriting the first operand (gds bit) of
DS_WRITE_B32 with the second operand (value to write).  This meant that
any time the value to write was stored in an odd number VGPR, the gds
bit would be set causing the instruction to write to GDS instead of LDS.

llvm-svn: 188522