Whatever the rights and wrongs of the way aggregate_value_p
handles call-preserved registers, it's a de facto part of the ABI,
so we shouldn't change it.  The patch simply extends the current
approach to whatever call-preserved set the function happens to
be using.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* function.c (aggregate_value_p): Work out which ABI the
	function is using before testing which registers are at least
	partly preserved by a call.

From-SVN: r276322