Revert "JVM IR: Implement CHECK_NOT_NULL as a lowering"

This reverts commit dcd72b06d8.

Using a temporary variable has an effect on casts and GC.

compiler/testData/codegen/box/casts/objectToPrimitiveWithAssertion.kt

@@ -0,0 +1,8 @@
+// IGNORE_BACKEND: JS, JS_IR, WASM
+@Suppress("UNCHECKED_CAST")
+fun <T> f() = 1L as T
+
+fun box(): String {
+    val x: Int = f()!! // T = Int?, but the cast succeeds because it's immediately upcasted to Number
+    return if (x == 1) "OK" else "fail: $x"
+}