Reunify x86 stack checking implementation

Reunify x86 stack checking implementation

The stack clash protection mechanism in the x86 back-end was implemented
by largely duplicating the existing stack checking implementation.  Now
the only significant difference between them is the probing window, which
is shifted by 1 probing interval (not 2 as documented in explow.c), but we
can certainly do 1 more probe for stack checking even if it is redundant
in almost all cases.

gcc/ChangeLog:
	* config/i386/i386.c (ix86_compute_frame_layout): Minor tweak.
	(ix86_adjust_stack_and_probe): Delete.
	(ix86_adjust_stack_and_probe_stack_clash): Rename to above and add
	PROTECTION_AREA parameter.  If it is true, probe PROBE_INTERVAL plus
	a small dope beyond SIZE bytes.
	(ix86_emit_probe_stack_range): Use local variable.
	(ix86_expand_prologue): Adjust calls to ix86_adjust_stack_and_probe
	and tidy up the stack checking code.
	* explow.c (get_stack_check_protect): Fix head comment.
	(anti_adjust_stack_and_probe_stack_clash): Likewise.
	(allocate_dynamic_stack_space): Add comment.

	* tree-nested.c (lookup_field_for_decl): Set the DECL_IGNORED_P and
	TREE_NO_WARNING but not TREE_ADDRESSABLE flags on the field.