compiler: stack allocate non-escaping makeslice

compiler: stack allocate non-escaping makeslice

    If we're making a slice of constant size that does not need to
    escape, allocate it on stack.

    In lower, do not create temporaries for constant size makeslice,
    so that it is easier to detect the slice is constant size later.

    Reviewed-on: https://go-review.googlesource.com/85636

From-SVN: r256404