[AMDGPU] Add options for waitcnt pass debugging; add instr count in debug output.

[AMDGPU] Add options for waitcnt pass debugging; add instr count in debug output.
-amdgpu-waitcnt-forcezero={1|0}  Force all waitcnt instrs to be emitted as s_waitcnt vmcnt(0) expcnt(0) lgkmcnt(0)
-amdgpu-waitcnt-forceexp=<n>  Force emit a s_waitcnt expcnt(0) before the first <n> instrs
-amdgpu-waitcnt-forcelgkm=<n> Force emit a s_waitcnt lgkmcnt(0) before the first <n> instrs
-amdgpu-waitcnt-forcevm=<n>   Force emit a s_waitcnt vmcnt(0) before the first <n> instrs

Differential Revision: https://reviews.llvm.org/D40091

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320084 91177308-0d34-0410-b5e6-96231b3b80d8