optimizer: inline constant `String` return value

[aviatesk]

The purpose of is_inlineable_constant is to avoid code size increases from copying objects too much. Strings would be fine since they are generally copied around by reference only.

[oscardssmith]

should this only happen for short string?

[aviatesk]

I think String are (always?) copied by reference whatever size it has. Is this understanding correct, @Keno or @vtjnash ?

[Keno]

That's the theory of it, but we should make sure that when we're compressing or serializing IR, we don't accidentally copy the strings around.