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/testData/codegen/boxWithStdlib/boxingOptimization/foldRange.kt

@@ -0,0 +1,9 @@
+import kotlin.test.assertEquals
+
+fun box(): String {
+    val result = (1..5).fold(0) { x, y -> x + y }
+
+    assertEquals(15, result)
+
+    return "OK"
+}