[flow analysis] Do not promote to mutual subtypes.

Previously, flow analysis had the rule that type promotion only
occurred when the type being tested was a subtype of the previously
promoted type (or the declared type, if there was no previous
promotion). This led to counterintuitive behaviors when the type being
tested and the previously promoted type were mutual subtypes (see
https://github.com/dart-lang/language/issues/4368).

With this change, the rule is updated so that type promotion only
occurs when the type being tested is a subtype of the previously
promoted type _and_ the previously promoted type is _not_ a subtype of
the type being tested. The user-visible difference is that promotion
to a mutual subtype no longer occurs.

This change makes flow analysis easier to reason about, and improves
its behavior in corner cases, but I believe it will have minimal
impact on real-world code. But to reduce the risk to existing code,
the change only takes effect when the `sound-flow-analysis` language
feature is enabled.

Fixes https://github.com/dart-lang/language/issues/4368.

Bug: https://github.com/dart-lang/language/issues/4368
Change-Id: I30dab017e043e75603d618df721c8a2683667cd5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/429200
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>