Fix ICE in verify_sra_access_forest

Fix ICE in verify_sra_access_forest

This fixes an issue with reverse storage order in SRA, which is caught
by the built-in verifier in verify_sra_access_forest.  The problem is
that propagate_subaccesses_from_rhs changes the type of an access
from aggregate to scalar and, as discussed elsewhere, this must be
done with extra care in the presence of reverse storage order.

gcc/ChangeLog
	* tree-sra.c (propagate_subaccesses_from_rhs): When a non-scalar
	access on the LHS is replaced with a scalar access, propagate the
	TYPE_REVERSE_STORAGE_ORDER flag of the type of the original access.

gcc/testsuite/ChangeLog
	* gnat.dg/opt85.ads, gnat.dg/opt85.adb: New test.