(SSA Refactor) Assert in acir gen after flattening pass merges two values

[jfecher]

Aim

Compiling code with if's with conditions that are witnesses:

fn main(a: bool, b: u32, c: u32) -> u32 {
    if a { b } else { c }
}

Expected Behavior

Expected the code to compile

Bug

The compiler panics with

Message:  assertion failed: `(left == right)`
  left: `Unsigned { bit_size: 1 }`,
 right: `Unsigned { bit_size: 32 }`: lhs and rhs types in Binary { lhs: Id(7176), rhs: Id(7183), operator: Mul } are not the same
Location: crates/noirc_evaluator/src/ssa_refactor/acir_gen/mod.rs:484

since the desugaring into a*b + (!a)*c, a and b are different types in the multiplication. a is a bool and b is a u32.

This affects: 9_conditional, 8_arrays, and regression after #1767 is merged.

To Reproduce

Installation Method

None

Nargo Version

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Additional Context

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