fix(ssa refactor): Fix ssa-gen of nested ifs

[jfecher]

Description

Problem*

Fixes a problem where the experimental ssa IR would crash while compiling nested if statements.

Although the crash was during function inlining, the error was actually the ssa-gen pass generated malformed SSA IR that was later given as input to inlining. The issue was when codegening if statements we would compile the then branch, then switch to compile the else branch, then switch back to the then branch to set its terminator. This is invalid if the then branch itself contains several blocks as we'd be setting the terminator of the first block rather than the final block. This caused nested if statements to be essentially erased from the program, but it was still possible to refer to their block arguments from blocks that are unreachable, which triggers the assert during function inlining.

Resolves #1392

Summary*

The fix is to change the ordering of ssa-gen for if statements slightly to terminate the then branch before switching to the else branch so that we do not need to switch back.

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.