[AMDGPU] Mark SMEM cache invalidations as not reading memory

This brings the MachineInstrs in line with the corresponding intrinsics
which have side effects but do not access memory. It also matches how
BUF cache invalidation instructions are defined.

The lit test changes are just because the machine scheduler previously
treated them like loads, and added an artificial scheduling edge from
them to the exit SU, which caused them to be scheduled earlier.

Differential Revision: https://reviews.llvm.org/D126074