FIR CFG: when unifying flows, group statements by assignment

Consider a function `run2` that has 2 lambda arguments called in place.
We don't know the order in which they're called, so here:

    var x: Any? = something
    run2(
      { x = null },
      { x as String },
    )
    // <--

it's not correct to simply `&&` the statements together, as that would
produce `x is Nothing? && x is String && x is Any?`. Instead, statements
should be grouped by assignment first, and different groups are `||`-ed.
This means in the above example we now get `x is Nothing? || (x is Any?
&& x is String)` == `x is String?`.

compiler/fir/analysis-tests/testData/resolve/cfg/flowFromInplaceLambda2.kt

@@ -73,6 +73,6 @@ fun test7(x: String?) {
             1,
             run { p.length; 123 } // Bad (p = null)
         )
-        p.length // Bad (p = null)
+        p<!UNSAFE_CALL!>.<!>length // Bad (p = null)
     }
 }