[AMDGPU] Refactor waitcnt encoding

- Refactor bit packing/unpacking
- Calculate bit mask given bit shift and bit width
- Introduce function for decoding bits of waitcnt
- Introduce function for encoding bits of waitcnt
- Introduce function for getting waitcnt mask (instead of using bare numbers)
- Introduce function fot getting max waitcnt(s) (instead of using bare numbers)

Differential Revision: https://reviews.llvm.org/D25298

llvm-svn: 283919