[IR][IPSCCP] Treat different function type as address taken (PR54258)

Without opaque pointers, this code currently treats a call through
a bitcast as the function being address taken, and IPSCCP relies
on this for correctness. Match the same behavior under opaque
pointers by checking that the function types are the same.

Fixes https://github.com/llvm/llvm-project/issues/54258.