Fix CFE legacy type promotion.

The previous implementation of type promotion in the CFE was unsound,
didn't follow the spec, and contained a lot of unnecessary logic
related to experiments that eventually led to flow analysis.  This
change updates type promotion to be nearly spec compliant and removes
most of the unnecessary logic.

The lack of spec compliance went (astonishingly) unnoticed for over
three years, probably mostly due to the fact that for the most part it
promoted more eagerly than it should; as a result, any code that was
erroneously accepted by the CFE was rejected by the analyzer.
Unfortunately, in at least one case the erroneous promotion led to
incorrect VM behavior (issue #43640).

There's one aspect in which the CFE implementation is still not spec
compliant.  The spec says that when analyzing an expression `e1 &&
e2`, if `e2` shows that a variable `v` has type `T`, but `e1` assigns
to `v`, then promotion does not occur.  This turns out to be difficult
to model in the existing architecture, since much of the type
promotion logic happens during parsing, and any assignments that occur
in `e1` are parsed prior to figuring out that a logical and is in
progress.  Fortunately, this is a rare circumstance, and allowing type
promotion in this situation is sound, so it should not lead to
incorrect VM behavior.  Considering how long the other incorrect logic
(which was much more severe) went unnoticed, it seems unlikely that it
will cause problems for users.

Bug: https://github.com/dart-lang/sdk/issues/43640
Change-Id: If97293733e64e286780de5079a703717b8f6f142
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/166040
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>