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

[Matt Arsenault via cfe-commits]

arsenm wrote:

> I suspect the long-term change to change the default IR to assume convergent will take some time as it will impact many subprojects. 

Turns out not really, I ran spec with this about 2 years ago and the only non-noise change was a mild improvement 

> Would you be OK with me patching the several DCE functions to not drop convergence intrinsics instead? It would solve this issue in practice, allowing us to move forward with addrspace cast legalization.

This is still trying to fix this in a roundabout way. You should be stopping the strip of the convergent attribute, not the intrinsic uses that happen to be in the function 

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