Make calls.c use function_arg_info internally

Make calls.c use function_arg_info internally

This patch makes the two main calls.c argument-processing
routines track the state of the argument in a function_arg_info
instead of using separate mode variables.

2019-08-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* calls.c (emit_library_call_value_1): Merge arg and orig_arg
	into a single function_arg_info, updating its fields when we
	apply pass-by-reference and promotion semantics.  Use the
	function_arg_info to track the mode rather than keeping it in
	a separate local variable.
	(initialize_argument_information): Likewise.  Base the final
	arg_to_skip on this new function_arg_info rather than creating
	a new one from scratch.

From-SVN: r274706