FIR: Fix incorrect type of block generated for inc operator

Previously, it was obtained from expected type of a variable being assigned,
but it's better to use the type of resulting expression

Initially this part was brought in 4ab0897d7d,
but as we see in commit message and tests it was all about unit-coercion

compiler/testData/diagnostics/tests/operatorsOverloading/incForAssignmentSmartCast.fir.kt

@@ -0,0 +1,13 @@
+// SKIP_TXT
+var c = 1
+
+fun nullable(): Int? = null
+
+fun foo(): Int {
+    var x = nullable()
+    if (x == null) {
+        x = c++
+    }
+
+    return x
+}