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.fir.txt

@@ -131,7 +131,7 @@ FILE: flowFromInplaceLambda2.kt
                     ^ Int(123)
                 }
                 ))
-                R|<local>/p|.R|kotlin/String.length|
+                R|<local>/p|.<Inapplicable(UNSAFE_CALL): kotlin/String.length>#
             }
         }