fix[venom]: alloca for default arguments

[charles-cooper]

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this commit fixes an `ir_node_to_venom` translation bug. when there is
a default argument to an external function, it can generate multiple
allocas, because the entry points allocate separate symbol tables, but
actually they should all correspond to the same alloca. for instance,
`external 1 foo(uint256)12345` and `external 1 foo()67890` both feed
into the same `external 1 foo()__common`, but the current translator
mistakenly creates different symbol tables for the two "feeder" entry
points, resulting in separate allocas for the same logical variable.

this commit fixes the bug by fusing the symbol tables for multiple
entry points to the same external function.

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