Restrict :c to commutative ops as intended

Restrict :c to commutative ops as intended

genmatch was supposed to restrict :c to verifiable commutative
operations while leaving :C to the "I know what I'm doing" case.
The following enforces this, cleaning up parsing and amending
the commutative_op helper.  There's one pattern that needs adjustment,
the pattern optimizing fmax (x, NaN) or fmax (NaN, x) to x since
fmax isn't commutative.

	* genmatch.cc (commutative_op): Add paramter to indicate whether
	all compares should be considered commutative.  Handle
	hypot, add_overflow and mul_overflow.
	(parser::parse_expr): Simplify 'c' handling by using
	commutative_op and error out when the operation is not.
	* match.pd ((minmax:c @0 NaN@1) -> @0): Use :C, we know
	what we are doing.