Migrator: Fix if-null promotion with GLB

Fixes https://github.com/dart-lang/sdk/issues/42375

EdgeBuilder wishes to do its own GLB type-joining, as does FlowAnalysis.
When FlowAnalysis pushes a promotion in ifNullExpression_rightBegin,
and then in write, and then pops those two promotions to join them in
ifNullExpression_end, EdgeBuilder is not given a chance to make its
own GLB node. Instead promotion never happens: the DecoratedType which
is the variable's type is promoted to non-nullable, for the left side.
The DecoratedType for the right side expression is not considered
non-nullable, so the FlowAnalysis.join will always fail to promote.

TypeOperations.adjustPromotedTypes solves this. It is a no-op in all
implementations except in EdgeBuilder.

Additionally FlowAnalysis.write's promotion fails for the same reason:
the only type of interest is the declared type made non-nullable, and
no matter what, the writtenType is not a subtype of this.

TypeOperations.forcePromotion solves this. It is a no-op in all
implementations except EdgeBuilder.

Change-Id: Iba6b9a69583af8fe1aa23124c7d66047acfff495
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152104
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>