fix wrong cursor inference in the context of loops

[zerbina]

Summary

• fix cursor inference bug where variables were erroneously inferred as cursors, leading to use-after-free issues
• improve cursor inference. var/let bindings are now inferred to be cursors more reliably

Fixes https://github.com/nim-works/nimskull/issues/1385

Details

For var/let bindings defined in loops, the aliveEnd is now only updated during the first analysis of the loop. This ensures that variables defined outside the loop, but only assigned to within the loop, always have longer alive times than variables defined within loops, thus preventing outer variables borrowing from inner variables (cursorfication cannot happen when a variable outlives its assignment source).

This fix also renders the isConditionallyReassigned heuristic obsolete, which was used to fix a more specific case of the same bug. The heuristic disabled cursorfication for all variables of which re-assignments are enclosed by a loop and if/else/elif/of, which also applied to, e.g.:

while cond:
  if cond:
    var a = newString(...)
    var b = newString(...)
    var x: string
    x = a
    x = b # this assignment disabled `x` being turned into a cursor
    discard a

With the heuristic removed, in the above example, x is now turned into a cursor, matching what would happen when there is no enclosing loop or if.

[saem]

/merge

[github-actions[bot]]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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