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

[Nathan Gauër via cfe-commits]

Keenuts wrote:

miss-typed enter, sent the comment above too early.

There are multiple ways to solve this issue:
 - fix the FunctionAttr to run DCE again once the `convergent` attribute is removed to make sure those invalid cases don't happen.
 - fix the FunctionAttr to not remove `convergent` if the function is used in a convergent call (the linked PR)
 - fix DCE to now remove convergence intrinsics, thus preventing FunctionAttr to wrongly remove the convergence attribute because the presence on such intrinsic is a sign the function needs it.

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