[frange] Handle relations in LTGT_EXPR.

[frange] Handle relations in LTGT_EXPR.

LTGT_EXPR hasn't been handling relations, especially with NANs as a
possibility.  This handles them while documenting how relations work
in a world with NANs.

Basically we need to special case VREL_EQ before calling
frelop_early_resolve.  Note that VREL_EQ on entry to a range-op entry
is really VREL_EQ U NAN, but to make sure about the NAN possibility,
one must look at the operands.  However, even VREL_EQ U NAN is false
for LTGT_EXPR since the latter is just NE_EXPR without a NAN.

After we handle VREL_EQ, we drop down to frelop_early_resolve
pretending to be a NE_EXPR, and everything should just map correctly.

2023-08-28  Aldy Hernandez  <aldyh@redhat.com>

	* range-op-float.cc (fold_range): Handle relations.