[vm/compiler] Do not create intersection types in type propagation.

AssertAssignable::ComputeType used to compute an intersection type
between cid available in the type of the value and abstract destination
type. This lead to creation of confusing impossible types like
{cid: kDoubleCid, type: int} and lead to problems in subsequent optimization
passes.

Fixes flutter/flutter#35613

Change-Id: I9662c2dd0deaabbbb73ddde30cc4ee32235bad67
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/108276
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Vyacheslav Egorov <[email protected]>

Author: mraleph

runtime/vm/compiler/backend/type_propagator.cc

@@ -1112,7 +1112,7 @@ CompileType AssertAssignableInstr::ComputeType() const {
     return *value_type;
   }
 
-  return CompileType::Create(value_type->ToCid(), dst_type());
+  return CompileType::FromAbstractType(dst_type(), value_type->is_nullable());
 }
 
 bool AssertAssignableInstr::RecomputeType() {