Compiler could elide many more postblit constructor calls

[TommiT]

Disclaimer: The "discovery" I'm about to describe here seems so 
obvious that I'm inclined to think that I've made some mistake.

The sole purpose of postblit constructors is to provide value 
semantics to structs which have mutable indirection (variables 
which have only immutable indirection have value semantics 
implicitly). Variables that are const/immutable can't have 
mutable indirection. Therefore, when making a copy from a 
const/immutable variable to a const/immutable variable, there's 
no need to call the postblit constructor.

Example:

----
struct S
{
     int[] values;

     this(this)
     {
         values = values.dup;
     }
}

void foo(const S) { }

void main()
{
     const S s;
     foo(s); // No need to call postblit
}