compiler: improvement in wasteful jmp elimination.

* stdlib/compiler.tl (compiler optimize): After the dataflow-driven
peephole optimization, call elim-dead-code again.

* stdlib/optimize.tl (basic-blocks check-bypass-empty): New method.
(basic-bocks elim-dead-code): After eliminating unreachable blocks
from the list, we use check-bypass-empty to squeeze out any
empty blocks: blocks that have no instructions in their list,
other than the leading label. This helps elim-next-jmp
to find more opportunities to eliminate a wasteful jump, because
sometimes these jumps straddle over empty blocks.
Furthermore, elim-next-jmp can generate more empty blocks itself;
so we check for this situation, delete the blocks and iterate.

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