[llvm] [SPIR-V] Add pass to merge convergence region exit targets (PR #92531)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> @Keenuts Thank you for the patch and detailed explanation! LGTM! A high level note for either this/future patch: It may be worth to update the [SPIRVUsage document](https://llvm.org/docs/SPIRVUsage.html) with how target intrinsics are affected in the Vulkan environment. We can discuss if there is any sensible way to split this document to discuss differences between SPIR-V "flavors".

Oh right, yes, could be valuable to add this, and in general, explain how the LLVM IR is structurized for the graphical SPIR-V.

https://github.com/llvm/llvm-project/pull/92531