[AMDGPU] Restore the s_memtime instruction in gfx1030

gfx1030 added a new way to implement readcyclecounter using the
SHADER_CYCLES hardware register, but the s_memtime instruction still
exists, so the MC layer should still accept it and the
llvm.amdgcn.s.memtime intrinsic should still work.

Differential Revision: https://reviews.llvm.org/D97928