[MCA] [AMDGPU] Adding an implementation to AMDGPUCustomBehaviour for handling...

[MCA] [AMDGPU] Adding an implementation to AMDGPUCustomBehaviour for handling s_waitcnt instructions.

This commit also makes some slight changes to the scheduling model for AMDGPU to set the RetireOOO flag for all scheduling classes.

This flag is only used by llvm-mca and allows instructions to retire out of order.

See the differential link below for a deeper explanation of everything.

Differential Revision: https://reviews.llvm.org/D104730