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/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/BoxingInterpreter.java

@@ -210,6 +210,10 @@ public class BoxingInterpreter extends OptimizationBasicInterpreter {
     @Override
     @NotNull
     public BasicValue merge(@NotNull BasicValue v, @NotNull BasicValue w) {
+        if (v == BasicValue.UNINITIALIZED_VALUE || w == BasicValue.UNINITIALIZED_VALUE) {
+            return BasicValue.UNINITIALIZED_VALUE;
+        }
+
         if (v instanceof BoxedBasicValue && ((BoxedBasicValue) v).typeEquals(w)) {
             onMergeSuccess((BoxedBasicValue) v, (BoxedBasicValue) w);
             return v;

compiler/backend/src/org/jetbrains/kotlin/codegen/optimization/boxing/RedundantBoxingMethodTransformer.java

@@ -114,6 +114,7 @@ public class RedundantBoxingMethodTransformer extends MethodTransformer {
             if (CollectionsKt.any(usedValues, new Function1<BasicValue, Boolean>() {
                 @Override
                 public Boolean invoke(BasicValue input) {
+                    if (input == BasicValue.UNINITIALIZED_VALUE) return false;
                     return input == null ||
                            !(input instanceof BoxedBasicValue) ||
                            !((BoxedBasicValue) input).isSafeToRemove() ||