gimplify.c (gimplify_modify_expr): When assigning to volatiles, copy the src value and return a copy.

	gcc/
	* gimplify.c (gimplify_modify_expr): When assigning to volatiles,
	copy the src value and return a copy.
	* doc/extend.texi (Volatiles): Move from C++ to C and expand.
	(C++ Volatiles): Adjust to describe C++ semantics only.

	gcc/testsuite/
	* gcc.target/i386/volatile-2.c: New.

From-SVN: r163400