fix(ssa refactor): Fix flattening pass inserting loads before stores occur

[jfecher]

Description

Problem*

Resolves #1756

Summary*

When an allocate instruction occurred in only one branch of an if expression, the flattening pass would still try to create a load instruction before the first store to the allocation in an attempt to save its previous (non-existent) value to merge later. This pass adds a field to track these local allocations and avoid merging stores for them since there is no value to merge in the opposite branch of the if.

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[ludamad]

LGTM but of course Kev has more SSA context