IRMover: Steal arguments when moving functions, NFC
Instead of copying arguments from the source function to the
destination, steal them. This has a few advantages.
- The ValueMap doesn't need to be seeded with (or cleared of) Arguments.
- Often the destination function won't have created any arguments yet, so this avoids malloc traffic.
- Argument names don't need to be copied.
Because argument lists are lazy, this required a new