[PR24021] Implement PLUS_EXPR range-op entry for floats.

This is the range-op entry for floating point PLUS_EXPR.  It's the
most intricate range entry we have so far, because we need to keep
track of rounding and target FP formats.  This will be the last FP
entry I commit, mostly to avoid disturbing the tree any further, and
also because what we have so far is enough for a solid VRP.

So far we track NANs and signs correctly.  We also handle relationals
(symbolics and numeric), both ordered and unordered, ABS_EXPR and
NEGATE_EXPR which are used to fold __builtin_isinf, and __builtin_sign
(__builtin_copysign is coming up).  All in all, I think this provide
more than enough for basic VRP on floats, as well as provide a basis
to flesh out the rest if there's interest.

My goal with this entry is to provide a template for additional binary
operators, as they tend to follow a similar pattern: handle NANs, do
the arithmetic while keeping track of rounding, and adjust for NAN.  I
may abstract the general parts as we do for irange's fold_range and
wi_fold.

	PR tree-optimization/24021

gcc/ChangeLog:

	* range-op-float.cc (propagate_nans): New.
	(frange_nextafter): New.
	(frange_arithmetic): New.
	(class foperator_plus): New.
	(floating_op_table::floating_op_table): Add PLUS_EXPR entry.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp-float-plus.c: New test.