Merge aarch64-cc-fusion into late-combine

I'd added the aarch64-specific CC fusion pass to fold a PTEST
instruction into the instruction that feeds the PTEST, in cases
where the latter instruction can set the appropriate flags as a
side-effect.

Combine does the same optimisation.  However, as explained in the
comments, the PTEST case often has:

  A: set predicate P based on inputs X
  B: clobber X
  C: test P

and so the fusion is only possible if we move C before B.
That's something that combine currently can't do (for the cases
that we needed).

The optimisation was never really AArch64-specific.  It's just that,
in an all-too-familiar fashion, we needed it in stage 3, when it was
too late to add something target-independent.

late-combine adds a convenient place to do the optimisation in a
target-independent way, just as combine is a convenient place to
do its related optimisation.

gcc/
	* config.gcc (aarch64*-*-*): Remove aarch64-cc-fusion.o from
	extra_objs.
	* config/aarch64/aarch64-passes.def (pass_cc_fusion): Delete.
	* config/aarch64/aarch64-protos.h (make_pass_cc_fusion): Delete.
	* config/aarch64/t-aarch64 (aarch64-cc-fusion.o): Delete.
	* config/aarch64/aarch64-cc-fusion.cc: Delete.
	* late-combine.cc (late_combine::optimizable_set): Take a set_info *
	rather than an insn_info * and move destination tests from...
	(late_combine::combine_into_uses): ...here. Take a set_info * rather
	an insn_info *.  Take the rtx set.
	(late_combine::parallelize_insns, late_combine::combine_cc_setter)
	(late_combine::combine_insn): New member functions.
	(late_combine::m_parallel): New member variable.
	* rtlanal.cc (pattern_cost): Handle sets of CC registers in the
	same way as comparisons.

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