[AMDGPU] Use s_cmp instead of s_cmpk

Don't bother pre-shrinking "s_cmp_lg_u32 reg, 0" to s_cmpk_lg_u32
because 0 is already an inline constant so the s_cmpk form is no
smaller.

This is just for consistency with the surrounding code and to simplify a
downstream patch.

Differential Revision: https://reviews.llvm.org/D138993