[AMDGPU] Added 'A' constraint for inline assembler

Summary: 'A' constraint requires an immediate int or fp constant that can be inlined in an instruction encoding.
This is the second part of the change. The llvm part has been committed as b087b91c.
See https://reviews.llvm.org/D78494

Reviewers: arsenm, rampitec

Differential Revision: https://reviews.llvm.org/D79493