Do not erase static profile by 0 autofdo profile

This patch makes auto-fdo more careful about keeping info we have
from static profile prediction.

If all counters in function are 0, we can keep original auto-fdo profile.
Having all 0 profile is not very useful especially becuase 0 in autofdo is not
very informative and the code still may have been executed in the train run.
I added comment about adding GUESSED_GLOBAL0_AFDO which would still preserve
info that the function is not hot in the profile, but I would like to do this
incrementally.

If function has non-zero counters, we can still keep info about zero being
reliable from static prediction (i.e. after EH or with cold attribute).

gcc/ChangeLog:

	* auto-profile.cc (update_count_by_afdo_count): New function.
	(afdo_set_bb_count): Add debug output; only set count if it is
	non-zero.
	(afdo_find_equiv_class): Add debug output.
	(afdo_calculate_branch_prob): Fix formating.
	(afdo_annotate_cfg): Add debug output; do not erase static
	profile if autofdo profile is all 0.

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