KT-36336 @EnhancedNullability and null checks

Don't insert implicit null check on a value of @EnhancedNullability type
used where @EnhancedNullability type is expected.

This uncovers a bunch of other problems in FE and BE.
KT-36343 and KT-36347 are bugs in StrictJavaNullabilityAssertions
implementation which should most likely be fixed in next major language
version (with proper breaking change notice).
KT-36344 is a design problem which should be addressed after 1.4 issues
are resolved.

compiler/ir/ir.psi2ir/src/org/jetbrains/kotlin/psi2ir/transformations/InsertImplicitCasts.kt

@@ -326,10 +326,10 @@ internal class InsertImplicitCasts(
                 else
                     implicitCast(expectedType, IrTypeOperator.IMPLICIT_DYNAMIC_CAST)
 
-            (valueType.isNullabilityFlexible() && valueType.containsNull()) && !expectedType.containsNull() ->
+            valueType.isNullabilityFlexible() && valueType.containsNull() && !expectedType.acceptsNullValues() ->
                 implicitNonNull(valueType, expectedType)
 
-            (valueType.hasEnhancedNullability() && !isLambdaReturnValue) && !expectedType.containsNull() ->
+            (valueType.hasEnhancedNullability() && !isLambdaReturnValue) && !expectedType.acceptsNullValues() ->
                 implicitNonNull(valueType, expectedType)
 
             KotlinTypeChecker.DEFAULT.isSubtypeOf(valueType, expectedType.makeNullable()) ->
@@ -359,6 +359,9 @@ internal class InsertImplicitCasts(
         }
     }
 
+    private fun KotlinType.acceptsNullValues() =
+        containsNull() || hasEnhancedNullability()
+
     private fun KotlinType.hasEnhancedNullability() =
         generatorExtensions.enhancedNullability.hasEnhancedNullability(this)