[vm/nnbd] Change type of sentinel values to Never

Special sentinel values which are used for lazy initialization can be
stored in variables of any type. In NNBD world Null is no longer a
bottom type as null cannot be assigned into variables of non-nullable
types. The new universal bottom type is Never, so type of sentinel
values is changed from Null to Never.

This also allows to make late fields non-nullable in the type
propagation and avoid extra updates to field guards.

This change also adds special handling of comparisons with sentinel
values into constant propagation as it can constant fold comparisons
of values with different cids.

Fixes https://github.com/dart-lang/sdk/issues/40796

Change-Id: I8266cfba0cc434b78ffa9570c30d434d4380d4af
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/143864
Reviewed-by: Liam Appelbe <liama@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Alexander Markov <alexmarkov@google.com>