[clang] [llvm] [IR] Mark convergence intrins as has-side-effect (PR #134844)

[Sameer Sahasrabuddhe via cfe-commits]

ssahasra wrote:

To take this to its logical conclusion, when convergence tokens are in use, the `convergent` attribute is redundant. All we need is a `noconvergent` attribute for function declarations. A function definition is convergent iff the body contains a call to the `entry` intrinsic, and a function declaration is assumed to be convergent unless it has a `noconvergent` attribute on it. So the immediate impact on the specification is that everywhere including the verifier, it is always okay to ignore the `convergent` attribute on a function definition if convergence tokens are in use.

https://github.com/llvm/llvm-project/pull/134844