RTL: Also support HOST_WIDE_INT with int iterators

RTL: Also support HOST_WIDE_INT with int iterators

Add wide integer aka 'w' rtx format support to int iterators so that
machine description can iterate over `const_int' expressions.

This is made by expanding standard integer aka 'i' format support,
observing that any standard integer already present in any of our
existing RTL code will also fit into HOST_WIDE_INT, so there is no need
for a separate handler.  Any truncation of the number parsed is made by
the caller.  An assumption is made however that no place relies on
capping out of range values to INT_MAX.

Now the 'p' format is handled explicitly rather than being implied by
rtx being a SUBREG, so actually assert that it is, just to play safe.

	gcc/
	* read-rtl.c: Add a page-feed separator at the start of iterator
	code.
	(struct iterator_group): Change the return type to HOST_WIDE_INT
	for the `find_builtin' member.  Likewise the second parameter
	type for the `apply_iterator' member.
	(atoll) [!HAVE_ATOQ]: Reorder.
	(find_mode, find_code): Change the return type to HOST_WIDE_INT.
	(apply_mode_iterator, apply_code_iterator)
	(apply_subst_iterator): Change the second parameter type to
	HOST_WIDE_INT.
	(find_int): Handle input suitable for HOST_WIDE_INT output.
	(apply_int_iterator): Rewrite in terms of explicit format
	interpretation.
	(rtx_reader::read_rtx_operand) <'w'>: Fold into...
	<'i', 'n', 'p'>: ... this.
	* doc/md.texi (Int Iterators): Document 'w' rtx format support.