Support empty if-statements

Type-check "if (...) ;" to Unit, report "implicit cast to Unit", propagate data
flow info out of its condition

 #KT-2478 Fixed

compiler/tests/org/jetbrains/jet/checkers/JetDiagnosticsTestGenerated.java

@@ -1565,6 +1565,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), "org.jetbrains.jet.generators.tests.GenerateTests", new File("compiler/testData/diagnostics/tests/controlStructures"), Pattern.compile("^(.+)\\.kt$"), true);
             }
             
+            @TestMetadata("emptyIf.kt")
+            public void testEmptyIf() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/controlStructures/emptyIf.kt");
+            }
+            
             @TestMetadata("forLoopWithNullableRange.kt")
             public void testForLoopWithNullableRange() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/controlStructures/forLoopWithNullableRange.kt");
@@ -1757,6 +1762,11 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 doTest("compiler/testData/diagnostics/tests/dataFlow/CalleeExpression.kt");
             }
             
+            @TestMetadata("EmptyIf.kt")
+            public void testEmptyIf() throws Exception {
+                doTest("compiler/testData/diagnostics/tests/dataFlow/EmptyIf.kt");
+            }
+            
             @TestMetadata("IsExpression.kt")
             public void testIsExpression() throws Exception {
                 doTest("compiler/testData/diagnostics/tests/dataFlow/IsExpression.kt");
@@ -3917,7 +3927,7 @@ public class JetDiagnosticsTestGenerated extends AbstractDiagnosticsTestWithEage
                 public void testContains() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inline/binaryExpressions/contains.kt");
                 }
-
+                
                 @TestMetadata("mathOperations.kt")
                 public void testMathOperations() throws Exception {
                     doTest("compiler/testData/diagnostics/tests/inline/binaryExpressions/mathOperations.kt");

compiler/tests/org/jetbrains/jet/codegen/generated/BlackBoxCodegenTestGenerated.java

@@ -1427,6 +1427,11 @@ public class BlackBoxCodegenTestGenerated extends AbstractBlackBoxCodegenTest {
             doTest("compiler/testData/codegen/box/controlStructures/doWhileFib.kt");
         }
         
+        @TestMetadata("emptyIf.kt")
+        public void testEmptyIf() throws Exception {
+            doTest("compiler/testData/codegen/box/controlStructures/emptyIf.kt");
+        }
+        
         @TestMetadata("finallyOnEmptyReturn.kt")
         public void testFinallyOnEmptyReturn() throws Exception {
             doTest("compiler/testData/codegen/box/controlStructures/finallyOnEmptyReturn.kt");