[llvm] [IR] Don't mark experimental.guard as willreturn (PR #69433)

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annamthomas wrote:

thank you for fixing this! After discussing with our team, we'll put up an RFC in a month or so (it would give some time for the "widenable.condition" work to bake in) to remove the experimental.guard support upstream. This should avoid maintainer burden and also save some compile time since we will no longer have to work with guards in passes.

https://github.com/llvm/llvm-project/pull/69433