AMDGPU/SDAG: Custom SETCC (i.e. ballot) is always uniform

The AMDGPUISD::SETCC node is like ISD::SETCC, but returns a lane mask
instead of a per-lane boolean. The lane mask is uniform.

This improves instruction selection for code patterns like
ctpop(ballot(x)), which can now use an S_BCNT1_* instruction instead
of V_BCNT_*.

GlobalISel already selects scalar instructions (an earlier commit
added a test case)..

Differential Revision: https://reviews.llvm.org/D123432