Add handling for negated conditions when folding IfElse instructions.

[TomAFrench]

In #6875 we added an optimisation such that we'd flatten the below nested if statement such that it became the simple if-statement below.

// Let then_condition_1 == then_condition_2

if then_condition_1 {
   then_value_1
} else if else_condition_1 {
   if then_condition_2 {
     then_value_1
   } else if else_condition_2 {
     else_value
   }
}

// Propagate conditions such that

if then_condition {
   then_value_1
} else if else_condition_1 {
     else_value
}

In theory we could perform a similar simplification for the case below:

Let then_condition_1 == else_condition_2

if then_condition_1 {
   then_value_1
} else if else_condition_1 {
   if then_condition_2 {
     then_value_1
   } else if else_condition_2 {
     else_value
   }
}

// Propagate conditions such that

if then_condition {
   then_value_1
} else if else_condition_1 {
    then_value_1
}

This is hindered by the fact that the else_conditions for if-else instructions in Noir's SSA are constructed on the fly so that they're very unlikely to share a valueid with a different value in the circuit. When we make this more reliable we can enable this optimization.