K2: Put boolean DFA variables under a feature flag

^KT-62547 Fixed

I also tested the commit by disabling DfaBooleanVariables in all tests
and checking what tests will fail

I didn't find anything that worked in K1, but doesn't work in K2 with
DfaBooleanVariables disabled

Related tests:
- DiagnosticCompilerTestFirTestdataTestGenerated$Resolve$Smartcasts
- LLFirPreresolvedReversedDiagnosticCompilerFirTestDataTestGenerated$Resolve$Smartcasts
- PreFirIdeSpecTestGenerated$NotLinked$Dfa
- DiagnosticCompilerTestFE10TestdataTestGenerated$Tests$SmartCasts
- FirPsiDiagnosticTestGenerated$Resolve$Expresssions.testSyntheticSmartCast

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/DiagnosticCompilerTestFE10TestdataTestGenerated.java

@@ -34962,6 +34962,12 @@ public class DiagnosticCompilerTestFE10TestdataTestGenerated extends AbstractDia
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {

analysis/low-level-api-fir/tests/org/jetbrains/kotlin/analysis/low/level/api/fir/diagnostic/compiler/based/LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated.java

@@ -34962,6 +34962,12 @@ public class LLFirPreresolvedReversedDiagnosticCompilerFE10TestDataTestGenerated
                 runTest("compiler/testData/diagnostics/tests/smartCasts/dataFlowInfoForArguments.kt");
             }
 
+            @Test
+            @TestMetadata("disableDfaBooleanVariables.kt")
+            public void testDisableDfaBooleanVariables() throws Exception {
+                runTest("compiler/testData/diagnostics/tests/smartCasts/disableDfaBooleanVariables.kt");
+            }
+
             @Test
             @TestMetadata("doubleLambdaArgument.kt")
             public void testDoubleLambdaArgument() throws Exception {