recog: Tweak insn_gen_fn::operator() definition

recog: Tweak insn_gen_fn::operator() definition

Fixes a “left operand of comma has no effect” warning that some were
seeing.  Also fixes a spurious ellipsis that Jonathan Wakely pointed
out.

2020-06-17  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* coretypes.h (first_type): New alias template.
	* recog.h (insn_gen_fn::operator()): Use it instead of a decltype.
	Remove spurious “...” and split the function type out into a typedef.