Front-end fix: result type corrected for increment / decrement expressions. Smart cast allowed on a postfix increment result. See KT-7561.

Now result type is receiver type for postfix increment, or increment result type for prefix increment.
A set of relevant tests.

compiler/testData/diagnostics/tests/smartCasts/kt7561.kt

@@ -0,0 +1,15 @@
+// Test for a potential byte code mistake for a postfix operation on a smart casted variable
+public fun box() : Int {
+    var i : Int? 
+    i = 10
+    val ii: Int = <!DEBUG_INFO_SMARTCAST!>i<!>
+    // k also should be Int
+    val k : Int = <!DEBUG_INFO_SMARTCAST!><!DEBUG_INFO_SMARTCAST!>i<!>++<!>
+    // KT-7561: both i and i++ should be Int, otherwise VerifyError can arise here
+    // VerifyError reason: byte code tries to store (i++) result which is Int (smart cast)
+    // into a j which is Int?
+    val j = <!DEBUG_INFO_SMARTCAST!>i<!>++
+    // and m also
+    val m = ++<!DEBUG_INFO_SMARTCAST!>i<!>
+    return <!DEBUG_INFO_SMARTCAST!>j<!> + k + m + <!DEBUG_INFO_SMARTCAST!>i<!> + ii
+}