Fix checking failure in IPA-SRA

Fix checking failure in IPA-SRA

This is a regression present on the mainline and 10 branch: on the one
hand, IPA-SRA does *not* disqualify accesses with zero size but, on the
other hand, it checks that accesses present in the tree have a (strictly)
positive size, thus trivially yielding an ICE in some cases.

gcc/ChangeLog:
	* ipa-sra.c (verify_access_tree_1): Relax assertion on the size.

gcc/testsuite/ChangeLog:
	* gnat.dg/opt91.ads, gnat.dg/opt91.adb: New test.
	* gnat.dg/opt91_pkg.ads, gnat.dg/opt91_pkg.adb: New helper.