[AMDGPU] Add llvm.amdgcn.cs.chain intrinsic to IR & verifier

We only check a subset of the constraints in the verifier:
* that we only call the intrinsic from functions with a restricted set of
calling conventions
* that the 'flags' argument is an immediate

Other checks are (probably) more appropriate for codegen.

Differential Revision: https://reviews.llvm.org/D151995