compiler: move some escape check to Mark_address_taken

compiler: move some escape check to Mark_address_taken

    Move some check of escape state earlier, from get_backend to
    Mark_address_taken. So we can reclaim escape analysis Nodes
    before kicking off the backend (not done in this CL). Also it
    makes it easier to check variables and closures do not escape
    when the escape analysis is run for the runtime package (also
    not done in this CL).

    Reviewed-on: https://go-review.googlesource.com/85735

From-SVN: r256406