close #2946 by ensuring let and const aren't put out of blocks

lib/compress.js

@@ -3115,11 +3115,18 @@ merge(Compressor.prototype, {
         return self;
     });
 
+    function can_be_extracted_from_if_block(node) {
+        return !(
+            node instanceof AST_Const ||
+            node instanceof AST_Let
+        );
+    }
+
     OPT(AST_BlockStatement, function(self, compressor){
         tighten_body(self.body, compressor);
         switch (self.body.length) {
           case 1:
-            if (!compressor.has_directive("use strict") && compressor.parent() instanceof AST_If
+            if (!compressor.has_directive("use strict") && compressor.parent() instanceof AST_If && can_be_extracted_from_if_block(self.body[0])
                 || can_be_evicted_from_block(self.body[0])) {
                 return self.body[0];
             }

test/compress/blocks.js

@@ -187,3 +187,30 @@ issue_1672_if_strict: {
     expect_stdout: true
     node_version: ">=6"
 }
+
+issue_2946_else_const: {
+    input: {
+        if (1) {
+            const x = 6;
+        } else {
+            const y = 12;
+        }
+        if (2) {
+            let z = 24;
+        } else {
+            let w = 48;
+        }
+    }
+    expect: {
+        if (1) {
+            const x = 6;
+        } else {
+            const y = 12;
+        }
+        if (2) {
+            let z = 24;
+        } else {
+            let w = 48;
+        }
+    }
+}