[AMDGPU] Force qsads instrs to use different dest register than source registers

The V_MQSAD_PK_U16_U8, V_QSAD_PK_U16_U8, and V_MQSAD_U32_U8 take more than 1 pass in hardware. For these three instructions, the destination registers must be different than all sources, so that the first pass does not overwrite sources for the following passes.

Differential Revision: https://reviews.llvm.org/D33783

llvm-svn: 304998