[FIR] Properly support smartcasts on stable when subjects in when conditions

^KT-49860 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/controlStructures/whenSubjectExpression.kt

@@ -0,0 +1,18 @@
+// DUMP_CFG
+// ISSUE: KT-49860
+
+fun whenWithSubjectExpression(x: Any) {
+    when (x) {
+        !is Double -> -1
+        0.0 -> 0 // `subj` in `subj == 0.0` must have type 'double'
+        else -> x.toInt()
+    }
+}
+
+fun whenWithSubjectVariable(x: Any) {
+    when (val y = x) {
+        !is Double -> -1
+        0.0 -> 0 // `subj` in `subj == 0.0` must have type 'double'
+        else -> y.toInt()
+    }
+}