[Attributor] Remove restriction on simplifying function pointers

Dropping this restriction seems to work fine (there are no assertion
failures), so it appears that either the updater got smarter or the
problematic cases are restricted elsewhere.

If doing this still causes issues, then the place to address it
would probably be https://github.com/llvm/llvm-project/blob/8f5bdaf481c3f4e6876a5ae0e4d7c6ed2042e6a6/llvm/lib/Transforms/IPO/Attributor.cpp#L1856-L1859,
which already prevents replacement outside the SCC, so I'm not
quite sure what this check is intended to avoid.

Differential Revision: https://reviews.llvm.org/D120987