[MCA] Adding an AMDGPUCustomBehaviour implementation.

This implementation allows mca to model the desired behaviour of the s_waitcnt
instruction. This patch also adds the RetireOOO flag to the AMDGPU instructions
within the scheduling model. This flag is only used by mca and allows
instructions to finish out-of-order which helps mca's simulations more closely
model the actual device.

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