Implement ABS_EXPR operator for frange.

Implement ABS_EXPR operator for frange.

Implementing ABS_EXPR allows us to fold certain __builtin_inf calls
since they are expanded into calls to involving ABS_EXPR.

This is an adaptation of the integer version.

gcc/ChangeLog:

	* range-op-float.cc (class foperator_abs): New.
	(floating_op_table::floating_op_table): Add ABS_EXPR entry.

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/vrp-float-abs-1.c: New test.