FIR DFA: store stability in RealVariable

As part of this change, we also extend the usage of RealVariable in more
places during DFA. Now mutable properties, property with custom getters,
delegated properties, etc are also treatd as a `RealVariable`. In
general this is needed in order to carry out smartcast computation in
order to report `SMARTCAST_IMPOSSIBLE`. It seems to also have side
effects that improves behavior of some test files.

compiler/testData/diagnostics/tests/inference/nestedCalls/makeNullableIfSafeCall.fir.kt

@@ -14,13 +14,13 @@ fun test(u: A?, x: A?, y: A?, z: A?, w: A, v: A?) {
     u!!.b<!UNNECESSARY_SAFE_CALL!>?.<!>foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     x?.b!!.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     // x?.b is not null
-    x!!.b<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    x<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.b<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
 
     y?.nb?.foo()!!
     y!!.nb?.foo()!!
     z?.nb!!.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     // z?.nb is not null
-    z!!.nb!!.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
+    z<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.nb!!.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
 
     w.b<!UNNECESSARY_SAFE_CALL!>?.<!>foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>
     w.b<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>.foo()<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!>

compiler/testData/diagnostics/tests/smartCasts/localDelegatedPropertyAfter.fir.kt

@@ -10,6 +10,6 @@ fun failsWithClassCastException() {
     val sometimesNotInt: Any? by AlternatingDelegate()
 
     if (sometimesNotInt is Int) {
-        sometimesNotInt.inc()
+        sometimesNotInt.<!UNRESOLVED_REFERENCE!>inc<!>()
     }
-}
+}

compiler/testData/diagnostics/tests/smartCasts/localDelegatedPropertyBefore.fir.kt

@@ -10,6 +10,6 @@ fun failsWithClassCastException() {
     val sometimesNotInt: Any? by AlternatingDelegate()
 
     if (sometimesNotInt is Int) {
-        sometimesNotInt.inc()
+        sometimesNotInt.<!UNRESOLVED_REFERENCE!>inc<!>()
     }
-}
+}

compiler/testData/diagnostics/tests/smartCasts/publicVals/otherModule.fir.kt

@@ -17,7 +17,7 @@ import a.X
 public fun X.gav(): Int {
     if (x != null)
         // Smart cast is not possible if definition is in another module
-        return x.length
+        return x<!UNSAFE_CALL!>.<!>length
     else
         return 0
 }
@@ -29,7 +29,7 @@ package a
 public fun X.gav(): Int {
     if (x != null)
         // Even if it's in the same package
-        return x.length
+        return x<!UNSAFE_CALL!>.<!>length
     else
         return 0
 }