[AMDGPU] Enhance s_waitcnt insertion before barrier for gfx12 (#90595)

[AMDGPU] Enhance s_waitcnt insertion before barrier for gfx12 (#90595)

Code to determine if a waitcnt is required before a barrier instruction
only
considered S_BARRIER.
gfx12 adds barrier_signal/wait so need to enhance the existing code to
look for
a barrier start (which is just an S_BARRIER for earlier architectures).