[AMDGPU] Add functions for composing and decomposing S_WAIT_DEPCTR operands

Add functions AMDGPU::DepCtr::encodeField*() and AMDGPU::DepCtr::decodeField*()
for each of vm_vsrc, va_vdst and sa_sdst. These are now used in
AMDGPUInsertDelayAlu and GCNHazardRecognizer so as to make working with
S_WAITCNT_DEPCTR operands easier and more readable.

Differential Revision: https://reviews.llvm.org/D154424