genemit: Add a generator struct

genemit: Add a generator struct

gen_exp now has quite a few arguments that need to be passed
to each recursive call.  This patch turns it and related routines
into member functions of a new generator class, so that the shared
information can be stored in member variables.

This also helps to make later patches less noisy.

gcc/
	* genemit.cc (generator): New structure.
	(gen_rtx_scratch, gen_exp, gen_emit_seq): Turn into member
	functions of generator.
	(gen_insn, gen_expand, gen_split, output_add_clobbers): Update
	users accordingly.