PR c++/93173 - incorrect tree sharing.

My patch for 93033 wasn't sufficient to handle all the possible sharing
introduced by split_nonconstant_init, and it occurred to me that it would
make sense to use the same unsharing technique as unshare_body, namely
copy_if_shared.

	PR c++/93033
gcc/
	* gimplify.c (copy_if_shared): No longer static.
	* gimplify.h: Declare it.
gcc/cp/
	* cp-gimplify.c (cp_gimplify_init_expr, cp_gimplify_expr): Use
	copy_if_shared after cp_genericize_tree.
	* typeck2.c (split_nonconstant_init): Don't unshare here.

From-SVN: r280126