fix(ssa_fuzzer): fix panic if compiler removed return value

[defkit]

Description

Problem*

The following SSA

acir(inline) fn main f0 {
  b0(v0: i32, v1: i32, v2: u64, v3: i32, v4: u1, v5: u1, v6: u1):
    jmp b1()
  b1():
    v7 = or v0, v3
    v9 = mod v7, i32 0
    v10 = cast v4 as u8
    return v10
}

compiles into

acir(inline) predicate_pure fn main f0 {
  b0(v0: i32, v1: i32, v2: u64, v3: i32, v4: u1, v5: u1, v6: u1):
    v7 = or v0, v3
    v9 = mod v7, i32 0
    constrain u1 0 == u1 1, "attempt to calculate the remainder with a divisor of zero"
    unreachable
}

so the number of return values is 0

Summary*

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.

[defkit]

fn main f0 {
  b0(v0: u1, v1: u64, v2: Field, v3: u1, v4: i64, v5: u1, v6: u1):
    jmpif v6 then: b1, else: b2
  b1():
    v9 = div Field 0, Field 0
    jmp b4()
  b2():
    v8 = div Field 0, Field 0
    jmp b4()
  b3():
    return v2
  b4():
    jmp b3()
}

compiles into ACIR

acir(inline) predicate_pure fn main f0 {
  b0(v0: u1, v1: u64, v2: Field, v3: u1, v4: i64, v5: u1, v6: u1):
    enable_side_effects v6
    v8 = div Field 0, Field 0
    v9 = not v6
    enable_side_effects v9
    v10 = div Field 0, Field 0
    enable_side_effects u1 1
    return v2
}

and Brillig

brillig(inline) predicate_pure fn main f0 {
  b0(v0: u1, v1: u64, v2: Field, v3: u1, v4: i64, v5: u1, v6: u1):
    jmpif v6 then: b1, else: b2
  b1():
    v11 = div Field 0, Field 0
    constrain u1 0 == u1 1, "attempt to divide by zero"
    unreachable
  b2():
    v8 = div Field 0, Field 0
    constrain u1 0 == u1 1, "attempt to divide by zero"
    unreachable
}

So I need bigger fix

[TomAFrench]

This likely came in with #9211 so I'd imagine the best fix for this is to replace the an unreachable return terminator with a zeroed out terminator rather than unreachable. (edit: seems like not)

[defkit]

I no longer encounter panic on my corpus. @TomAFrench can I merge it?

[TomAFrench]

Go for it!