Cleanup irange::set.

Cleanup irange::set.

[This is actually part of a larger patch that actually changes
behavior, but I thought I'd commit the non-invasive cleanups first
which will simplify the upcoming work.]

irange::set was doing more work than it should for legacy ranges.
I cleaned up various unnecessary calls to swap_out_of_order_endpoints,
as well as some duplicate code that could be done with normalize_min_max.

I also removed an obsolete comment wrt sticky infinite overflows.
Not only did the -INF/+INF(OVF) code get removed in 2017,
but normalize_min_max() uses wide ints, which ignored overflows
altogether.

gcc/ChangeLog:

	* value-range.cc (irange::swap_out_of_order_endpoints): Rewrite
	into static function.
	(irange::set): Cleanup redundant manipulations.
	* value-range.h (irange::normalize_min_max): Modify object
	in-place instead of modifying arguments.