[AMDGPU][MC][GFX10] Added syntactic sugar for s_waitcnt_depctr operand

Added the following helpers:

    depctr_hold_cnt(...)
    depctr_sa_sdst(...)
    depctr_va_vdst(...)
    depctr_va_sdst(...)
    depctr_va_ssrc(...)
    depctr_va_vcc(...)
    depctr_vm_vsrc(...)

Differential Revision: https://reviews.llvm.org/D123022