compiler: fix crashes on cyclic var/type references

compiler: fix crashes on cyclic var/type references

    This patch fixes type traversal to avoid compiler crashes for test
    cases where a type T includes an expression that refers back to the
    type without actually explicitly mentioning T. Examples include

      var x [uintptr(unsafe.Sizeof(&x))]byte
      var a [len(a)]int

    The fix involves expanding the set of types that the traversal code
    "remembers" (to avoid cycles) to include array types, and introducing an
    additional guard in Builtin_call_expression::do_is_constant to catch
    cyclic type constructs.

    Fixes golang/go#25299
    Fixes golang/go#25679
    Fixes golang/go#25315
    Fixes golang/go#25680

    Reviewed-on: https://go-review.googlesource.com/115796

From-SVN: r261168