[InstCombine] Support opaque pointers in callee bitcast fold

To make this actually trigger, we also need to check whether the
function types differ, which is a hidden cast under opaque pointers.
The transform is somewhat less relevant there because it is
primarily about pointer bitcasts, but it can also happen with other
bit- or pointer-castable types.

Byval handling is easier with opaque pointers because there is no
need to adjust the byval type, we only need to make sure that it's
still a pointer.