[GlobalISel][CallLowering] Look through bitcasts from constant function pointers.

Calls to ObjC's objc_msgSend function are done by bitcasting the function global
to the required function type signature. This patch looks through this bitcast
so that we can do a direct call with bl on arm64 instead of using an indirect blr.

Differential Revision: https://reviews.llvm.org/D74241