fix(ssa refactor): Prevent stores in 'then' branch from affecting the 'else' branch

[jfecher]

Description

Problem*

Resolves #1826

Summary*

Previously, any stores in the then branch of an if would be picked up by the else branch as well (See the linked issue for more details). This PR adds an extra step to the flattening pass to restore the old values of each store between the then and else branches to fix this.

I've also altered the merge_stores_with_offset test since we no longer support offsets from allocations after the switch to immutable arrays. I changed the test to a simple merge stores test but with an else block.

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.