rs6000: Small improvement to the C++17 ABI fix [PR94707]

rs6000: Small improvement to the C++17 ABI fix [PR94707]

Anyway, based on IRC discussion with Richard Sandiford on IRC, we should
probably test type uids instead of type pointers because type uids aren't
reused, but type pointers in a very bad luck case could be, and having the
static var at filescope and GTY((deletable)) is an overkill (and with costs
during GC time).

2020-04-23  Jakub Jelinek  <jakub@redhat.com>

	PR target/94707
	* config/rs6000/rs6000-call.c (rs6000_discover_homogeneous_aggregate):
	Use TYPE_UID (TYPE_MAIN_VARIANT (type)) instead of type to check
	if the same type has been diagnosed most recently already.