this commit fixes a bug in the ir_node_to_venom translator. previously,
`ir_node_to_venom` tried to detect unique allocas based on
heuristics. this commit removes the heuristics and fixes the issue
in the frontend by passing through a unique ID for each variable in
the metadata. this ID is also passed into the `alloca` and `palloca`
instructions to aid with debugging. note that this results in improved
code, presumably due to more allocas being able to be reified.

this commit makes a minor change to the `sqrt()`, builtin, which is to
use `z_var.as_ir_node()` instead of `z_var.pos`, since `.as_ir_node()`
correctly tags with the alloca metadata. to be maximally conservative,
we could branch, only using `z_var.as_ir_node()` if we are using
the venom pipeline, but the change should be correct for the legacy
pipeline as well anyways.