[FIR] Save info that when was used as expression

compiler/fir/analysis-tests/testData/resolve/whenWithWhenAsStatement.fir.txt

@@ -0,0 +1,16 @@
+FILE: whenWithWhenAsStatement.kt
+    public final fun test(value: R|kotlin/Int|): R|kotlin/Unit| {
+        when (R|<local>/value|) {
+            ==($subj$, Int(0)) ->  {
+            }
+            ==($subj$, Int(1)) ->  {
+                when (R|<local>/value|) {
+                    ==($subj$, Int(2)) ->  {
+                        Boolean(false)
+                    }
+                }
+
+            }
+        }
+
+    }

compiler/fir/analysis-tests/testData/resolve/whenWithWhenAsStatement.kt

@@ -0,0 +1,8 @@
+fun test(value: Int) {
+    when (value) {
+        0 -> {}
+        1 -> when (value) {
+            2 -> false
+        }
+    }
+}