Add a type_checked_load_relative to support relative function pointer tables

[aschwaighofer]

Summary

This adds a type_checked_load_relative intrinsic whose semantics it is to
load a relative function pointer.

A relative function pointer is a pointer to a 32bit value that when
added to its address yields the address of the function.

[kubamracek]

Can we also get a test with llvm.type.checked.load.relative where the call *doesn't* get devirtualized?

[aschwaighofer]

Include test case where the llvm.type.checked.load.relative intrinsic is
expanded to compute the pointer underlying the relative pointer value.