[GVN] Remove users from ICF when RAUWing loads

When performing store to load forwarding, replacing users of the
load may turn an indirect call into one with a known callee, in
which case it might become willreturn, invalidating cached ICF
information. Avoid this by removing users.

This is a bit more aggressive than strictly necessary (e.g. this
shouldn't be necessary when doing load-load CSE), but better safe
than sorry.

Fixes https://github.com/llvm/llvm-project/issues/48805.