slp: Add complex operations class to share first match among all matchers

This introduces a common class complex_operations_pattern which encapsulates
the complex add, mul, fma and fms pattern in such a way so that the first match
is shared.

gcc/ChangeLog:

	* tree-vect-slp-patterns.c (class complex_operations_pattern,
	complex_operations_pattern::matches,
	complex_operations_pattern::recognize,
	complex_operations_pattern::build): New.
	(slp_patterns): Use it.