Do not treat uninitialized value as a reason to retain boxing

See testData/simpleUnitializedMerge.kt
On exit from `if` we merge value in variable with slot 1 (x):
- from `if` body we get BoxedBasicValue
- from outer block we get UNITIALIZED_VALUE

So we just suppose `x` is unitialized after `if`
and there's no need to mark BoxedValue as unsafe to remove
because it's anyway can't be used after `if`

 #KT-6842 Fixed

compiler/tests/org/jetbrains/kotlin/codegen/BytecodeTextTestGenerated.java

@@ -376,6 +376,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("kt6842.kt")
+        public void testKt6842() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/kt6842.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("kt7224.kt")
         public void testKt7224() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/kt7224.kt");
@@ -412,6 +418,12 @@ public class BytecodeTextTestGenerated extends AbstractBytecodeTextTest {
             doTest(fileName);
         }
 
+        @TestMetadata("simpleUninitializedMerge.kt")
+        public void testSimpleUninitializedMerge() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/simpleUninitializedMerge.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("unsafeRemoving.kt")
         public void testUnsafeRemoving() throws Exception {
             String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/codegen/bytecodeText/boxingOptimization/unsafeRemoving.kt");