fix if_return on block-scoped variables (#2021)

fixes #1317

lib/compress.js

@@ -1068,7 +1068,9 @@ merge(Compressor.prototype, {
             }
 
             function can_merge_flow(ab) {
-                if (!ab) return false;
+                if (!ab || !all(ret, function(stat) {
+                    return !(stat instanceof AST_Const || stat instanceof AST_Let);
+                })) return false;
                 var lct = ab instanceof AST_LoopControl ? compressor.loopcontrol_target(ab) : null;
                 return ab instanceof AST_Return && in_lambda && is_return_void(ab.value)
                     || ab instanceof AST_Continue && self === loop_body(lct)

test/compress/if_return.js

@@ -326,3 +326,61 @@ issue_512: {
         }
     }
 }
+
+issue_1317: {
+    options = {
+        if_return: true,
+    }
+    input: {
+        !function(a) {
+            if (a) return;
+            let b = 1;
+            function g() {
+                return b;
+            }
+            console.log(g());
+        }();
+    }
+    expect: {
+        !function(a) {
+            if (a) return;
+            let b = 1;
+            function g() {
+                return b;
+            }
+            console.log(g());
+        }();
+    }
+    expect_stdout: "1"
+    node_version: ">=6"
+}
+
+issue_1317_strict: {
+    options = {
+        if_return: true,
+    }
+    input: {
+        "use strict";
+        !function(a) {
+            if (a) return;
+            let b = 1;
+            function g() {
+                return b;
+            }
+            console.log(g());
+        }();
+    }
+    expect: {
+        "use strict";
+        !function(a) {
+            if (a) return;
+            let b = 1;
+            function g() {
+                return b;
+            }
+            console.log(g());
+        }();
+    }
+    expect_stdout: "1"
+    node_version: ">=4"
+}