compiler: fix check for notinheap conversion

    
    A normal pointer may not be converted to a notinheap pointer.  We were
    erroneously permitting a conversion from a normal pointer to a
    notinheap unsafe.Pointer, which is useless since unsafe.Pointer is not
    marked notinheap.  Correct the test to permit a conversion from
    unsafe.Pointer to a notinheap pointer, which is the same test that the
    gc compiler uses.
    
    The test case for this is in the 1.9 runtime package.
    
    Reviewed-on: https://go-review.googlesource.com/62731

From-SVN: r252745