hir_analysis: skip self type of host effect preds in variances_of
Discovered as part of an implementation of rust-lang/rfcs#3729 - w/out this then when introducing const trait bounds: many more interesting tests change with different output, missing errors, new errors, etc related to this but they all depend on feature flags and are much more complex than this test.
r? ``@oli-obk``
cf8498d36a