[AddressLowering] Fix partial_apply arg indexing.
When rewriting arguments, the index used is into the callee's argument
list. For full applies, that is identical to the index into the
instruction's argument list. For partial applies, it is not.
Previously, though, the index was used as if it were an index into the
instruction's argument list to get an argument ref. Here, use instead
the newly added convenience to get the argument ref using the index into
the callee's arguments.
