value-range: update comments

One of the difficulties I ran into when familiarizing myself with
value-range.{h,cc} is that the comments and classes refer to
representations of "ranges", but the implementation has grown beyond
mere ranges of values (such as with bitmasks and NaN-tracking).

Arguably "range" could refer to the mathematical definition: the set
of possible outputs of a function, but I find it much clearer to think
of these classes as efficient representations of subsets of possible
values of a type.

This patch updates various leading comments in a way that clarifies
the intent of these classes (for me, at least).

gcc/ChangeLog:
	* value-range.cc (irange_bitmask::irange_bitmask): Fix typo in
	comment.
	* value-range.h (class vrange): Update leading comment to refer
	to "subsets" rather than "ranges".  Allude to the available
	subclasses.  Warn that the classes can be over-approximations and
	thus can introduce imprecision.
	(class irange_bitmask): Updating leading comment to refer to
	knowledge about a "value", rather than a "range".  Reword
	description of MASK and VALUE to clarify implementation, and
	add an example.
	(class irange): Update leading comment to refer to a
	"subset of possible values" rather than a "range", and
	that subclasses have responsibility for storage.
	(class nan_state): Rewrite leading comment.
	(class frange final): Update leading comment to refer to
	subsets of possible values, rather than ranges, and to
	consistently use "Nan" when capitalizing.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>

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