[AMDGPU] Waitcnt pass: add debug options

- Add "amdgpu-waitcnt-forcezero" to force all waitcnt instrs to be emitted as s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)

- Add debug counters to control force emit of s_waitcnt instrs; debug counters:
si-insert-waitcnts-forceexp: force emit s_waitcnt expcnt(0) instrs
si-insert-waitcnts-forcevm: force emit s_waitcnt lgkmcnt(0) instrs
si-insert-waitcnts-forcelgkm: force emit s_waitcnt vmcnt(0) instrs

- Add some debug statements

Note that a variant of this patch was previously committed/reverted.

Differential Revision: https://reviews.llvm.org/D45888

llvm-svn: 330862