tree-sra.c (sra_walk_fns): Document new restriction.

tree-sra.c (sra_walk_fns): Document new restriction.

	* tree-sra.c (sra_walk_fns) <ldst>: Document new restriction.
	(sra_walk_modify_expr) <rhs_elt>: Treat the reference as a use
	if the lhs has side-effects.
	<lhs_elt>: Treat the reference as a use if the rhs has side-effects.

From-SVN: r122492