[AMDGPU] Fix a miscompile with S_ADD/S_SUB

The helper function isBoolSGPR is too aggressive when determining
when a v_cndmask can be skipped on a boolean value because the
function does not check the operands of and/or/xor.

This can be problematic for the Add/Sub combines that can leave
bits set even for inactive lanes leading to wrong results.

Fix this by inspecting the operands of and/or/xor recursively.

Differential Revision: https://reviews.llvm.org/D86878