[llvm] [Local] Preserve !callees when merging calls (PR #225307)

[Mian Miftah via llvm-commits]

mmiftahx wrote:

> Separate the SimplifyCFG and MergeFunc fixes. SimplifyCFG looks fine at a glance. MergeFunc should be fixed by switching to combining metadata instead of comparing it, as we recently did for poison flags.

@nikic I've removed the MergeFunctions from this PR. I'll send a separate patch that combines metadata, following the poison-flag change.

https://github.com/llvm/llvm-project/pull/225307