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

[Sameer Sahasrabuddhe via llvm-commits]

ssahasra wrote:

- Rebased on top of main, which includes the SelectionDAG changes for convergence tokens.
- Eliminated the G_* opcodes in favour of the opcodes introduced for SelectionDAG

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