compiler: only check whether struct or array types are big

    
    Fetching the size of a type typically involves a hash table lookup,
    and is generally non-trivial.  The escape analysis code calls is_big
    more than one might expect.  So only fetch the size if we need it.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/197699

From-SVN: r276187