feat[venom]: improve liveness computation

[charles-cooper]

traversing in reverse topsort order improves stack scheduling slightly (codesize improvement of about 0.35%)

this commit also adds a topsort method to CFGAnalysis, and speeds it up by only checking the terminator instruction instead of iterating over all the instructions in every basic block.

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[charles-cooper]

marking as draft since this seems to introduce a scheduler regression elsewhere 😱

tests/functional/codegen/features/test_immutable.py:237: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

...

vyper/venom/venom_to_assembly.py:597: in dup
    assembly.append(_evm_dup_for(depth))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

depth = -16

    def _evm_dup_for(depth: int) -> str:
        dup_idx = 1 - depth
        if not (1 <= dup_idx <= 16):
>           raise StackTooDeep(f"Unsupported dup depth {dup_idx}")
E           vyper.exceptions.StackTooDeep: Unsupported dup depth 17
E           
E           This is an unhandled internal compiler error. Please create an issue on Github to notify the developers!
E           https://github.com/vyperlang/vyper/issues/new?template=bug.md

vyper/venom/venom_to_assembly.py:616: StackTooDeep