AMDGPU: really don't commute REV opcodes if the target variant doesn't exist

If pseudoToMCOpcode failed, we would return the original opcode, so operands
would be swapped, but the instruction would remain the same.
It resulted in LSHLREV a, b ---> LSHLREV b, a.

This fixes Glamor text rendering and
piglit/arb_sample_shading-builtin-gl-sample-mask on VI.

This is a candidate for stable branches.

v2: the test was simplified by Tom Stellard
llvm-svn: 240824