Rework type inference logic to avoid redundant constraint gathering.

Previously, the front end would perform downwards and upwards
inference using separate TypeConstraintGatherer objects.  This meant
that any constraint gathering work performed during downwards
inference had to be repeated during upwards inference.

This change avoids the extra work by using a single
TypeConstraintGatherer object for both downwards and upwards
inference.

This should help pave the way to implementing support for
https://github.com/dart-lang/language/issues/731 (improved inference
for fold etc.)

Change-Id: Ib4031ab1397d6a8547a705f386632de0e4dd1a2b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/241120
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>