Reduce startup costs for Value_Range.

Value_Range is our polymorphic temporary that can hold any range.  It
is used for type agnostic code where it isn't known ahead of time,
what the type of the range will be (irange, france, etc).  Currently,
there is a temporary of each type in the object, which means we need
to construct each range for every temporary.  This isn't scaling
well now that prange is about to add yet another range type.

This patch removes each range, opting to use in-place new for a byte
buffer sufficiently large to hold ranges of any type.  It reduces the
memory footprint by 14% for every Value_Range temporary (from 792 to
680 bytes), and we are guaranteed it will never again grow as we add
more range types (strings, complex numbers, etc).

Surprisingly, it improves VRP performance by 6.61% and overall
compilation by 0.44%, which is a lot more than we bargained for
when we started working on prange performance.

There is a slight change in semantics for Value_Range.  The default
constructor does not initialize the object at all.  It must be
manually initialized with either Value_Range::set_type(), or by
assigning a range to it.  This means that IPA's m_known_value_ranges
must be initialized at allocation, instead of depending on the empty
constructor to initialize it to VR_UNDEFINED for unsupported_range.

I have taken the time to properly document both the class, and each
method.  If anything isn't clear, please let me know so I can adjust it
accordingly.

gcc/ChangeLog:

	* ipa-fnsummary.cc (evaluate_properties_for_edge): Initialize Value_Range's.
	* value-range.h (class Value_Range): Add a buffer and remove
	m_irange and m_frange.
	(Value_Range::Value_Range): Call init.
	(Value_Range::set_type): Same.
	(Value_Range::init): Use in place new to initialize buffer.
	(Value_Range::operator=): Tidy.

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