Make FOR_EACH_IMM_USE_STMT work w/o fake imm use node

Make FOR_EACH_IMM_USE_STMT work w/o fake imm use node

This is an attempt to fix PR122502 by making a FOR_EACH_IMM_USE_FAST
with in an FOR_EACH_IMM_USE_STMT on _the same_ VAR work without
the former running into the FOR_EACH_IMM_USE_STMT inserted marker
use operand.  It does this by getting rid of the marker.

The downside is that this in principle restricts the set of operations
that can be done on the immediate use list of VAR.  Where previously
almost anything was OK (but technically not well-defined what happens
to the iteration) after this patch you may only remove immediate
uses of VAR on the current stmt from the FOR_EACH_IMM_USE_STMT
iteration.  In particular things will break if you happen to remove
the one immediate use of VAR on the stmt immediately following
the set of immediate uses on the currrent stmt.

Additional checking to combat such cases is implemented in a
followup.

	PR tree-optimization/122502
	* ssa-iterators.h (imm_use_iterator::iter_node): Remove.
	(imm_use_iterator::next_stmt_use): New.
	(next_readonly_imm_use): Adjust checking code.
	(end_imm_use_stmt_traverse): Simplify.
	(link_use_stmts_after): Likewise.  Return the last use
	with the same stmt.
	(first_imm_use_stmt): Simplify.  Set next_stmt_use.
	(next_imm_use_stmt): Likewise.
	(end_imm_use_on_stmt_p): Adjust.

	* gcc.dg/torture/pr122502-2.c: New testcase.