`if` function evaluates both values despite the condition

Inline conditional function of if(condition, then-value, else-value) has some unexpected behavior.

It tries to evaluate "else-value" even if condition is true and else-value in unneeded. In the same way, "then-value" is evaluated even if condition is false.

That may cause 'Operation on an invalid type' error if condition and value processing depends on value type.

.mymixin(@val) {
    myprop: if(isnumber(@val), @val * 42, @val);
}

a {
    .mymixin(11);    // works perfectly with the result of 462
}

b {
    .mymixin(null);  // causes 'Operation on an invalid type' error
}

less / less.js

`if` function evaluates both values despite the condition #3528