[AMDGPU] Assembler: SOP* instruction fixes

s_bitset0_b64, s_bitset1_b64 has 32-bit src0, not 64-bit.
s_rfe_b64 has just one destination operand and no source.
Uncomment S_BITCMP* and S_SETVSKIP, adjust SOPC_* classes for that.
Add s_memrealtime test and change comments in smem.s to follow common style.
Change test for s_memtime to use non-zero register to make it really test encoding.
Add tests for s_buffer_load*.
Add tests for SOPC instructions (same for SI and VI)

Differential Revision: http://reviews.llvm.org/D18040

llvm-svn: 263420