FIR CFG: handle reassignments with explicit receivers more precisely

When a looping control structure has a reassignment with an explicit
receiver, we don't need to erase its smartcast info as it has to be a
non-local property and is thus automatically unstable anyway. However,
we should still do that upon reaching the assignment itself for slightly
more precise error messages (as the property not just "could be"
reassigned, but *has been* reassigned already).

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

@@ -10,7 +10,7 @@ fun Box.test() {
             other.item = null
             take<String>(<!SMARTCAST_IMPOSSIBLE!>item<!>)
             this.item = null
-            take<String>(<!SMARTCAST_IMPOSSIBLE!>item<!>)
+            take<String>(<!ARGUMENT_TYPE_MISMATCH!>item<!>)
         }
     }
 
@@ -18,7 +18,7 @@ fun Box.test() {
     myRun {
         if (item != null) {
             this.item = null
-            take<String>(<!SMARTCAST_IMPOSSIBLE!>item<!>)
+            take<String>(item)
         }
     }
 }