[WebAssembly] Support opaque pointers in AddMissingPrototypes

The change here is basically the same as in D108880: Rather than
looking at bitcasts, look at calls and their function type. We
still need to look through bitcasts to find those calls.

The change in llvm/test/CodeGen/WebAssembly/add-prototypes-conflict.ll
is due to different visitation order. add-prototypes-opaque-ptrs.ll
is a copy of add-prototypes.ll with -force-opaque-pointers.

Differential Revision: https://reviews.llvm.org/D109256