SSA immediate use iterator checking

SSA immediate use iterator checking

The following implements additional checking around
SSA immediate use iteration.  Specifically this prevents

 - any nesting of FOR_EACH_IMM_USE_STMT inside another iteration
   via FOR_EACH_IMM_USE_STMT or FOR_EACH_IMM_USE_FAST when iterating
   on the same SSA name

 - modification (for now unlinking of immediate uses) of a SSA
   immediate use list when a fast iteration of the immediate uses
   of the SSA name is active

 - modification (for now unlinking of immediate uses) of the immediate
   use list outside of the block of uses for the currently active stmt
   of an ongoing FOR_EACH_IMM_USE_STMT of the SSA name

To implement this additional bookkeeping members are put into the
SSA name structure when ENABLE_GIMPLE_CHECKING is active.  I have
kept the existing consistency checking of the fast iterator.

	* ssa-iterators.h (imm_use_iterator::name): Add.
	(delink_imm_use): When in a FOR_EACH_IMM_USE_STMT iteration
	enforce we only remove uses from the current stmt.
	(end_imm_use_stmt_traverse): Reset current stmt.
	(first_imm_use_stmt): Assert no FOR_EACH_IMM_USE_STMT on
	var is in progress.  Set the current stmt.
	(next_imm_use_stmt): Set the current stmt.
	(auto_end_imm_use_fast_traverse): New, lower iteration
	depth upon destruction.
	(first_readonly_imm_use): Bump the iteration depth.
	* tree-core.h (tree_ssa_name::active_iterated_stmt,
	tree_ssa_name::fast_iteration_depth): New members when
	ENABLE_GIMPLE_CHECKING.
	* tree-ssanames.cc (make_ssa_name_fn): Initialize
	immediate use verifier bookkeeping members.