compiler: make sure variables captured by defer closure live

compiler: make sure variables captured by defer closure live

    Local variables captured by the deferred closure need to be live
    until the function finishes, especially when the deferred
    function runs. In Function::build, for function that has a defer,
    we wrap the function body in a try block. So the backend sees
    the local variables only live in the try block, without knowing
    that they are needed also in the finally block where we invoke
    the deferred function. Fix this by creating top-level
    declarations for non-escaping address-taken locals when there
    is a defer.

    An example of miscompilation without this CL:

    func F(fn func()) {
            didPanic := true
            defer func() {
                    println(didPanic)
            }()
            fn()
            didPanic = false
    }

    With escape analysis turned on, at optimization level -O1 or -O2,
    the store "didPanic = false" is elided by the backend's
    optimizer, presumably because it thinks "didPanic" is not live
    after the store, so the store is useless.

    Reviewed-on: https://go-review.googlesource.com/86241

From-SVN: r256706