Fix wrong SRA with VIEW_CONVERT_EXPR and reverse SSO

Fix wrong SRA with VIEW_CONVERT_EXPR and reverse SSO

Most cases of VIEW_CONVERT_EXPRs involving reverse scalar storage order are
disqualified for SRA because they are storage_order_barrier_p, but you can
still have a VIEW_CONVERT_EXPR to a regular composite type being applied to
a component of a record type with reverse scalar storage order.

In this case the bypass for !useless_type_conversion_p in sra_modify_assign,
albeit already heavily guarded, triggers and may generate wrong code, so the
patch makes sure that it does only when the SSO is the same on both side.

gcc/
	* tree-sra.cc (sra_modify_assign): Check that scalar storage order
	is the same on the LHS and RHS before rewriting one with the model
	of the other.
gcc/testsuite/
	* gnat.dg/sso17.adb: New test.