[llvm] [GlobalISel] convergence control tokens and intrinsics (PR #67006)

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

- rebased to upstream/main
- split out LLT::token() as a separate commit
- added support for tail calls along with a test
- removed redundant return (after an unreachable)

https://github.com/llvm/llvm-project/pull/67006