FIR DFA: check for reassignments of LHS when handling ?..

`x?.y != null` does not imply that `x != null` if e.g. an argument to
`y` has reassigned `x` in the meantime.

The same is true for `x == y` and `functionWithContract(x, y)`, but
those are somewhat harder to implement since there is no easy way to
find the last node of a certain argument.

^KT-55096

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosisCompilerTestFE10TestdataTestGenerated.java

@@ -30643,6 +30643,12 @@ public class DiagnosisCompilerTestFE10TestdataTestGenerated extends AbstractDiag
                     runTest("compiler/testData/diagnostics/tests/smartCasts/variables/reassignedDependency.kt");
                 }
 
+                @Test
+                @TestMetadata("reassignedInRhs.kt")
+                public void testReassignedInRhs() throws Exception {
+                    runTest("compiler/testData/diagnostics/tests/smartCasts/variables/reassignedInRhs.kt");
+                }
+
                 @Test
                 @TestMetadata("varAsUse.kt")
                 public void testVarAsUse() throws Exception {