Pseudocode: Generate proper instructions for when conditions. Eliminate unnecessary magics

compiler/testData/cfg/controlStructures/whenConditions.kt

@@ -0,0 +1,12 @@
+import java.util.Collections
+
+fun foo(a: Number) {
+    val t = when (a) {
+        1 -> "1"
+        in Collections.singleton(2) -> "2"
+        is Int -> "Int"
+        !in Collections.singleton(3) -> "!3"
+        !is Number -> "!Number"
+        else -> null
+    }
+}