[AMDGPU] Simplify setcc (sext from i1 b), -1|0, cc

Depending on the compare code that can be either an argument of
sext or negate of it. This helps to avoid v_cndmask_b64 instruction
for sext. A reversed value can be further simplified and folded into
its parent comparison if possible.

Differential Revision: https://reviews.llvm.org/D34545

llvm-svn: 306446