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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364831 91177308-0d34-0410-b5e6-96231b3b80d8