Adjust variable names and comments in value-query.*

Adjust variable names and comments in value-query.*

Now that range_of_expr can take arbitrary tree expressions, not just
SSA names or constants, the method names and comments are slightly out
of date.  This patch adjusts them to reflect reality.

gcc/ChangeLog:

	* value-query.cc (value_query::value_on_edge): Rename name to
	expr.
	(range_query::range_on_edge): Same.
	(range_query::value_of_expr): Same.
	(range_query::value_on_edge): Same.
	* value-query.h (class value_query): Same.
	(class range_query): Same.