AMDGPU/GlobalISel: Handle 16-bit SALU min/max

AMDGPU/GlobalISel: Handle 16-bit SALU min/max

This needs to be extended to s32, and expanded into cmp+select.  This
is relying on the fact that widenScalar happens to leave the
instruction in place, but this isn't a guaranteed property of
LegalizerHelper.

llvm-svn: 364831