psi2ir: fix nullability assertion generation for platform types

Incorporate PR from Steven Schäfer into IrType-based implicit cast
insertion (commit 17b925636e8717e7648c5d7b792c6ab4d18f776d).

NB this still uses originalKotlinType to determine if the type was
nullability flexible. It is somewhat error-prone and something we want
to get rid of. However, it boils down to some design questions related
to implicit null checks in Kotlin - e.g., it might be Ok to just treat
nullability flexible type `T!` as `T?` in IR, generate null checks for
all usages of type `T?` where a non-null type is expected, and later
eliminate the null checks that are redundant according to the (quite
conservative) criterion in the redundant null check elimination.

compiler/tests/org/jetbrains/kotlin/ir/IrTextTestCaseGenerated.java

@@ -1606,6 +1606,11 @@ public class IrTextTestCaseGenerated extends AbstractIrTextTestCase {
             KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("compiler/testData/ir/irText/types"), Pattern.compile("^(.+)\\.kt$"), TargetBackend.ANY, true);
         }
 
+        @TestMetadata("explicitEqualsAndCompareToCallsOnPlatformTypeReceiver.kt")
+        public void testExplicitEqualsAndCompareToCallsOnPlatformTypeReceiver() throws Exception {
+            runTest("compiler/testData/ir/irText/types/explicitEqualsAndCompareToCallsOnPlatformTypeReceiver.kt");
+        }
+
         @TestMetadata("intersectionType1_NI.kt")
         public void testIntersectionType1_NI() throws Exception {
             runTest("compiler/testData/ir/irText/types/intersectionType1_NI.kt");
@@ -1640,5 +1645,10 @@ public class IrTextTestCaseGenerated extends AbstractIrTextTestCase {
         public void testLocalVariableOfIntersectionType_NI() throws Exception {
             runTest("compiler/testData/ir/irText/types/localVariableOfIntersectionType_NI.kt");
         }
+
+        @TestMetadata("nullabilityAssertionOnExtensionReceiver.kt")
+        public void testNullabilityAssertionOnExtensionReceiver() throws Exception {
+            runTest("compiler/testData/ir/irText/types/nullabilityAssertionOnExtensionReceiver.kt");
+        }
     }
 }