Make memory copy functions scalar storage order barriers

Make memory copy functions scalar storage order barriers

This addresses the issue raised about the usage of memory copy functions
to toggle the scalar storage order.  Recall that you cannot (the compiler
errors out) take the address of a scalar which is stored in reverse order,
but you can do it for the enclosing aggregate type., which means that you
can also pass it to the memory copy functions.  In this case, the optimizer
may rewrite the copy into a scalar copy, which is a no-no.

gcc/c-family/ChangeLog:
	* c.opt (Wscalar-storage-order): Add explicit variable.

gcc/c/ChangeLog:
	* c-typeck.c (convert_for_assignment): If -Wscalar-storage-order is
	set, warn for conversion between pointers that point to incompatible
	scalar storage orders.

gcc/ChangeLog:
	* gimple-fold.c (gimple_fold_builtin_memory_op): Do not fold if
	either type has reverse scalar storage order.
	* tree-ssa-sccvn.c (vn_reference_lookup_3): Do not propagate through
	a memory copy if either type has reverse scalar storage order.

gcc/testsuite/ChangeLog:
	* gcc.dg/sso-11.c: New test.
	* gcc.dg/sso/sso.exp: Pass -Wno-scalar-storage-order.
	* gcc.dg/sso/memcpy-1.c: New test.