[AMDGPU] Mark time intrinsics as nomem, hassideeffects

Adding IntrHasSideEffects to @llvm.amdgcn.s.memtime and
@llvm.amdgcn.s.memrealtime means that we can stop pretending they read
and write memory, and similarly for the corresponding pseudo
instructions.

This should stop these intrinsics from being rescheduled past all other
instructions, even ones which don't load or store.

See also https://reviews.llvm.org/D58635.

Differential Revision: https://reviews.llvm.org/D115227