fix(ssa): Unused functions removals post folding constant Brillig calls

[vezenovm]

Description

Problem*

Silly bug leftover from #7260, but also fixes this removal pass to be more accurate. We are not checking the full new call graph of unused functions.

e.g. for the following SSA:

        g0 = Field 2

        acir(inline) fn main f0 {
          b0():
            v1 = call f1() -> Field
            return v1
        }

        brillig(inline) fn entry_point f1 {
          b0():
            v1 = call f2() -> Field
            return v1
        }

        brillig(inline) fn one f2 {
          b0():
            v1 = add g0, Field 3
            return v1
        }

After constant folding we would still have:

        brillig(inline) fn one f2 {
          b0():
            v1 = add g0, Field 3
            return v1
        }

even though it would be unused. This is because we would only inline Brillig calls for ACIR entry points. Thus, we would still have a call to f2, even though f1 is now unused, thus f2 is also unused.

Summary*

I simply moved to using our pre-existing remove_unreachable_functions pass. I felt this kept a better separation of concerns for the passes and was a more complete removal of unreachable functions.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

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

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: 152052b	Previous: 0b6c382	Ratio
noir-lang_schnorr_	1 s	0 s	+∞
noir-lang_noir_check_shuffle_	1 s	0 s	+∞
noir-lang_noir_base64_	2 s	1 s	2
noir-lang_eddsa_	2 s	1 s	2
noir-lang_ec_	1 s	0 s	+∞

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[asterite]

Makes sense and looks good!