JVM IR: Don't produce CHECKCASTs on null constants (KT-36650)

compiler/testData/codegen/bytecodeText/coroutines/cleanup/nullCleanup.kt

@@ -12,6 +12,12 @@ suspend fun test() {
     blackhole(a)
 }
 
-// jsut before suspension point
+// 2 PUTFIELD .*L\$0 : Ljava/lang/Object;
+
+// JVM_TEMPLATES:
+// just before suspension point
 // 1 ACONST_NULL
-// 2 PUTFIELD .*L\$0 : Ljava/lang/Object;
+
+// JVM_IR_TEMPLATES:
+// two stores to initialize the `a` variable and one null constant to store in the spill slot.
+// 3 ACONST_NULL