`ELSIF` conditional regression

[mhasel]

#1379 has resulted in a regression when evaluating ELSIF conditions if said condition contains a call to a function with aggregate return types. Due to the way the AST is lowered, these calls will now always be evaluated even if any of the previous conditional blocks already evaluated as TRUE, possibly leading to side-effects.

A possible solution to this is further lowering IF conditional blocks and nesting each ELSIF statement in a separate IF THEN ELSE statement within the previous conditions ELSE block, i.e.

IF condition THEN
    // ...
ELSIF condition2 THEN
    // ...
END_IF

should turn into

IF condition THEN
    // ...
ELSE
    IF condition2 THEN
        // ...
    ELSE
        // ...
    END_IF
END_IF