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/bytecodeText/boxingOptimization/kt6842.kt

@@ -0,0 +1,17 @@
+fun box(): Long {
+    val x1 = (1..5).map { it * 40 }
+    val x2 = (1..5).fold(0) { x, y -> x + y }
+    val x3 = (1..5).reduce { x, y -> x + y }
+    val x4 = (1..5).count { it > 0 }
+
+    val y1 = (1L..5L).map { it * 40L }
+    val y2 = (1L..5L).fold(0L) { x, y -> x + y }
+    val y3 = (1L..5L).reduce { x, y -> x + y }
+    val y4 = (1L..5L).count { it > 0L }
+
+    return (x1.first() + x2 + x3 + x4).toLong() + y1.first() + y2 + y3 + y4.toLong()
+}
+
+// 5 nextLong
+// 5 nextInt
+// 0 next\s*\(

compiler/testData/codegen/bytecodeText/boxingOptimization/simpleUninitializedMerge.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    var result = 0
+    if (1 == 1) {
+        val x: Int? = 1
+        result += x!!
+    }
+    return "OK"
+}
+
+// 0 java/lang/Integer.valueOf