Always check result from build_<COND> in range-op-float.cc

Always check result from build_<COND> in range-op-float.cc

A result of false from build_<COND> in range-ops means the result is
final and needs no further adjustments.  This patch documents this,
and changes all uses to check the result.  There should be no change
in functionality.

gcc/ChangeLog:

	* range-op-float.cc (build_le): Document result.
	(build_lt): Same.
	(build_ge): Same.
	(foperator_ge::op2_range): Check result of build_*.
	(foperator_unordered_le::op1_range): Same.
	(foperator_unordered_le::op2_range): Same.
	(foperator_unordered_gt::op1_range): Same.
	(foperator_unordered_gt::op2_range): Same.
	(foperator_unordered_ge::op1_range): Same.
	(foperator_unordered_ge::op2_range): Same.