compiler: only check whether struct or array types are big

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

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@276187 138bc75d-0d04-0410-961f-82ee72b054a4