[AMDGPU] Optimize waitcnt insertion for flat memory operations

Change waitcnt insertion to check the memory operand tokens to see if
flat memory operations access VMEM in the same way it does to check if
accessing LDS. This avoids adding waitcnt for counters for address
spaces that are not accessed.

In addition, only generate the pessimistic waitcnt 0 if a flat memory
operation appears to access both VMEM and LDS.

This benefits flat memory operations that explicitly specify the
address space as GLOBAL or LOCAL.

Differential Revision: https://reviews.llvm.org/D89618