[FIR] Fix detecting that if in then branch of outer if used as expression

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.fir.txt

@@ -0,0 +1,20 @@
+FILE: nestedIfInLambda.kt
+    public final fun run(block: R|() -> kotlin/Unit|): R|kotlin/Unit| {
+    }
+    public final fun test(b1: R|kotlin/Boolean|, b2: R|kotlin/Boolean|): R|kotlin/Unit| {
+        lvar result: R|kotlin/Boolean| = Boolean(false)
+        R|/run|(<L> = run@fun <anonymous>(): R|kotlin/Unit| {
+            when () {
+                R|<local>/b1| ->  {
+                    when () {
+                        R|<local>/b2| ->  {
+                            R|<local>/result| = Boolean(true)
+                        }
+                    }
+
+                }
+            }
+
+        }
+        )
+    }

compiler/fir/analysis-tests/testData/resolve/exhaustiveness/positive/nestedIfInLambda.kt

@@ -0,0 +1,10 @@
+fun run(block: () -> Unit) {}
+
+fun test(b1: Boolean, b2: Boolean) {
+    var result = false
+    run {
+        if (b1)
+            if (b2)
+                result = true
+    }
+}