predcom: Adjust some unnecessary update_ssa calls

As Richi suggested in PR100794, this patch is to remove
some unnecessary update_ssa calls with flag
TODO_update_ssa_only_virtuals, also do some refactoring.

Bootstrapped/regtested on powerpc64le-linux-gnu P9,
x86_64-redhat-linux and aarch64-linux-gnu, built well
on Power9 ppc64le with --with-build-config=bootstrap-O3,
and passed both P8 and P9 SPEC2017 full build with
{-O3, -Ofast} + {,-funroll-loops}.

gcc/ChangeLog:

	* tree-predcom.c (execute_pred_commoning): Remove update_ssa call.
	(tree_predictive_commoning_loop): Factor some cleanup stuffs into
	lambda function cleanup, remove scev_reset call, and adjust return
	value.
	(tree_predictive_commoning): Adjust for different changed values,
	only set flag TODO_update_ssa_only_virtuals if changed.
	(pass_data pass_data_predcom): Remove TODO_update_ssa_only_virtuals
	from todo_flags_finish.