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/bytecodeText/inline/inlineReturnsNothing1.kt

@@ -1,4 +1,5 @@
-// NB '!!' uses Intrinsics.throwNpe/checkNotNull
+// NB '!!' uses Intrinsics.throwNpe/checkNotNull, but IR follows it with ATHROW
+// while the old backend returns null from `exit`
 inline fun exit(): Nothing = null!!
 
 fun box(): String {
@@ -13,4 +14,7 @@ fun box(): String {
     return a
 }
 
+// JVM_TEMPLATES
 // 1 ATHROW
+// JVM_IR_TEMPLATES
+// 2 ATHROW

compiler/testData/codegen/bytecodeText/inline/inlineReturnsNothing3.kt

@@ -1,3 +1,5 @@
+// NB '!!' uses Intrinsics.throwNpe/checkNotNull, but IR follows it with ATHROW
+// while the old backend returns null from `exit`
 inline fun exit(): Nothing = null!!
 inline fun exita(): Nothing = exit() // ATHROW
 inline fun exitb(): Nothing = exita() // ATHROW
@@ -14,4 +16,7 @@ fun box(): String {
     return a
 }
 
-// 4 ATHROW
+// JVM_TEMPLATES
+// 4 ATHROW
+// JVM_IR_TEMPLATES
+// 5 ATHROW