fix corner case in conditionals (#4777)

lib/compress.js

@@ -11044,6 +11044,7 @@ merge(Compressor.prototype, {
             if (len != b.length) return -2;
             for (var i = 0; i < len; i++) {
                 if (!a[i].equivalent_to(b[i])) {
+                    if (a[i] instanceof AST_Spread !== b[i] instanceof AST_Spread) return -3;
                     for (var j = i + 1; j < len; j++) {
                         if (!a[j].equivalent_to(b[j])) return -2;
                     }

test/compress/spreads.js

@@ -85,7 +85,7 @@ collapse_vars_4: {
     node_version: ">=6"
 }
 
-conditionals_farg: {
+conditionals_farg_1: {
     options = {
         conditionals: true,
     }
@@ -107,6 +107,28 @@ conditionals_farg: {
     node_version: ">=6"
 }
 
+conditionals_farg_2: {
+    options = {
+        conditionals: true,
+        pure_getters: "strict",
+        reduce_vars: true,
+    }
+    input: {
+        var log = console.log;
+        (function(a) {
+            return a.length ? log(...a) : log("FAIL");
+        })([ "PASS" ]);
+    }
+    expect: {
+        var log = console.log;
+        (function(a) {
+            return a.length ? log(...a) : log("FAIL");
+        })([ "PASS" ]);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=6"
+}
+
 dont_inline: {
     options = {
         inline: true,