[vm] Store type class id for Types as ClassIdTagType, not Smi.

Now that Types are no longer created for top level classes, the type
class id is guaranteed to fit in ClassIdTagType. That means we can
reduce the size of Type objects slightly as well as avoid the need to
tag and untag the type class id.

Also fixes an issue uncovered in GenerateSubtype1TestCacheLookup where
the retrieved super type was not first checked to be non-null.

TEST=Refactoring, so existing tests.

Cq-Include-Trybots: luci.dart.try:vm-kernel-linux-release-ia32-try,vm-kernel-linux-release-x64-try,vm-kernel-nnbd-linux-release-simarm-try,vm-kernel-nnbd-linux-release-simarm64-try,vm-kernel-nnbd-linux-release-x64-try,vm-kernel-precomp-linux-release-simarm-try,vm-kernel-precomp-linux-release-simarm64-try,vm-kernel-precomp-linux-release-simarm_x64-try,vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-nnbd-linux-release-simarm64-try,vm-kernel-precomp-nnbd-linux-release-simarm_x64-try,vm-kernel-precomp-nnbd-linux-release-x64-try,vm-kernel-precomp-linux-debug-x64c-try,vm-kernel-linux-debug-x64c-try,vm-kernel-precomp-linux-debug-simarm64c-try
Change-Id: I851a827843e34eac6961e0f06f888b40bf16df55
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217221
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>