[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@283919
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