compiler: omit write barrier for assignment to *(convert(&local))

compiler: omit write barrier for assignment to *(convert(&local))

    Assignments to local variables don't need a write barrier. But
    currently the compiler inserts a write barrier if the LHS is a
    local variable with type converted, as *(convert(&local)). Let
    the compiler recognize this pattern and omit the write barrier.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/182541

From-SVN: r272550