frange: add both zeros to ranges when there's the possiblity of equality.

Every time there's equality at play, we must be careful that any
equality with zero matches both -0.0 and +0.0 when honoring signed
zeros.

We were doing this correctly for the == and != op1_range operators
(albeit inefficiently), but aren't doing it at all when building >=
and <=.  This fixes the oversight.

There is change in functionality here for the build_* functions.

This is the last "simple" patch I submit before overhauling NAN and
sign tracking.  And that will likely be after Cauldron because it will need
further testing (lapack, ppc64le, -ffinite-math-only, etc).

Regstrapped on x86-64 Linux, plus I ran selftests for
-ffinite-math-only.

gcc/ChangeLog:

	* range-op-float.cc (frange_add_zeros): New.
	(build_le): Call frange_add_zeros.
	(build_ge): Same.
	(foperator_equal::op1_range): Same.
	(foperator_not_equal::op1_range): Same.

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