compress duplicated variable definitions (#1817)

These are surprisingly common, as people reuse the same variable name within loops or switch branches.

test/compress/drop-unused.js

@@ -1029,3 +1029,30 @@ delete_assign_2: {
     }
     expect_stdout: true
 }
+
+drop_var: {
+    options = {
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a;
+        console.log(a, b);
+        var a = 1, b = 2;
+        console.log(a, b);
+        var a = 3;
+        console.log(a, b);
+    }
+    expect: {
+        console.log(a, b);
+        var a = 1, b = 2;
+        console.log(a, b);
+        a = 3;
+        console.log(a, b);
+    }
+    expect_stdout: [
+        "undefined undefined",
+        "1 2",
+        "3 2",
+    ]
+}

test/compress/reduce_vars.js

@@ -1639,7 +1639,7 @@ redefine_arguments_1: {
             return typeof arguments;
         }
         function g() {
-            return"number";
+            return "number";
         }
         function h(x) {
             var arguments = x;
@@ -1951,7 +1951,6 @@ pure_getters_2: {
         var a = a && a.b;
     }
     expect: {
-        var a;
         var a = a && a.b;
     }
 }