compiler,runtime: Determine if allocations need new pointers in runtime.

    
    As the removed comment states, if the package being compiled played
    certain tricks with pointers that looked like integers, the compiler
    might allocate space for new pointers unnecessarily.  Since the type
    information on the heap is now precise, this logic can be moved to the
    runtime.
    
    Reviewed-on: https://go-review.googlesource.com/11581

From-SVN: r225757