Refactor cfg_dataflow, cfg_liveness, cfgize

[gretay-js]

This issue is to keep track of CFG-related improvements identified during the review of PR#547.

• remove duplicate "is_pure" after Refactor and correct the "is pure" and "can raise" (port upstream PR#10354 and PR#10387) #555 merges
• liveness: Poptrap, Prolog, Pushtrap live across should be set to "value" not empty. See comment.
• forward: use less_equal not compare for fixpoint check
• backward: simplify special treatment of trap handler blocks. See comment.
• backward: change the types of transformers (don't return instruction) and the result (per instruction not per block)
• merge forward and backward if possible
• Cfgize.Trap_depth_and_exn : simplify exceptional_successor, it's always (pop stack) if the block can raise. See comment.
• reenable cfg_equivalence check for layout