[GlobalDCE] Handle relative pointers in VFE (for Swift vtables)

To support Virtual Function Elimination to Swift, this PR adds support for Swift
vtables which contain "relative pointers" instead of direct pointer references.
These are in the form of:

@symbol = ... {
  i32 trunc (i64 sub (i64 ptrtoint (<type> @target to i64), i64 ptrtoint (... @symbol to i64)) to i32)
}

The PR extends GlobalDCE's way of looking up a vtable offset into a dependency
to be able to see through this expression and find the target symbol.

Differential Revision: https://reviews.llvm.org/D107645