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

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
+    }
+}