remove all refops if a basic block is a raising block

[dlee992]

A possible way to
fix #1044

Notes for reviewers:

• remove all refops if a basic block is a raising block, since we allow memory leak in a raising block
• put this pruning operation in the per_bb pattern
• after per_bb pattern removes this case, fanout_raise pattern can do the rest as before

[dlee992]

Hi, @sklam. This is another small improvement for ref-pruning. It can enjoy your review! Thanks

[dlee992]

Hi, any comment for this PR and the related issue? cc @esc

[esc]

@dlee992 I suppose this ought to make typed.List faster? If so, do you have any benchmarks to share?

[dlee992]

Yeah, I will make up one test case for it ASAP (probably tmr).
In theory, it will definitely help the performance since we prune more atomic refcounts.