Integrate nonzero bits with irange.

The nonzero bits and integer ranges compliment each other quite well,
and it only makes sense to make the mask a first class citizen in the
irange.  We do a half assed job of keeping ranges and nonzero bits
somewhat in sync in SSA_NAME_RANGE_INFO, and the goal has always
been to integrate them properly.  This patch does that, in preparation
for streaming out full-resolution iranges between passes (think
SSA_NAME_RANGE_INFO).

Having nonzero bits in the irange allows us to get better results from
things like irange::contains_p() and keeping them in the irange allows
us to propagate the bits throughout with the ranger.  This patch
provides the bare infrastructure, without any optimizations to
range-ops, etc.  Those will come as follow-ups.

A few notes:

Legacy SSA_NAME_RANGE_INFO updates the nonzero bits every time a range
is set.  Here instead, we don't update the nonzero bits on a new
range, but calculate it on the fly when irange::get_nonzero_bits() is
called.  The goal is to only store nonzero bits that provide
meaningful information that can't be gleaned from the range itself.
But you can always call get_nonzero_bits() and get the full
information.

Nonzero bits are not supported in legacy mode.  The mask may be set
as a consequence of propagation or reading global ranges, but no
one from legacy land should be querying irange::get_nonzero_bits.  There
is an assert enforcing this.  However, legacy/global set_nonzero_bits()
continue to work as before.  There is no change to legacy behavior.

There is virtually no performance change with this patch, as there are
no consumers.  The next patch I post will be the SSA_NAME_RANGE_INFO
conversion to the new world, in which I will discuss performance
proper.  Hint: I'll be chewing up the time budget we gained with the
vrange conversion.

Tested and benchmarked on x86-64 Linux.

gcc/ChangeLog:

	* value-range-storage.cc (irange_storage_slot::set_irange): Set
	nonzero bits in irange.
	(irange_storage_slot::get_irange): Get nonzero bits from irange.
	* value-range.cc (irange::operator=): Set nonzero bits.
	(irange::irange_set): Same.
	(irange::irange_set_anti_range): Same.
	(irange::set): Same.
	(irange::verify_range): Same.
	(irange::legacy_equal_p): Check nonzero bits.
	(irange::equal_p): Same.
	(irange::contains_p): Handle nonzero bits.
	(irange::irange_union): Same.
	(irange::irange_intersect): Same.
	(irange::dump): Same.
	(irange::set_nonzero_bits): New.
	(irange::get_nonzero_bits): New.
	(irange::intersect_nonzero_bits): New.
	(irange::union_nonzero_bits): New.
	(irange::dump_bitmasks): New.
	* value-range.h (class irange): Add m_nonzero_mask.
	(gt_ggc_mx): Handle nonzero bits.
	(gt_pch_nx): Same.
	(irange::set_undefined): Set nonzero bits.
	(irange::set_varying): Same.
	(irange::normalize_kind): Call set_undefined.

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