fix corner case in dead_code (#5642)

fixes #5641

lib/compress.js

@@ -12706,13 +12706,23 @@ Compressor.prototype.compress = function(node) {
         return try_evaluate(compressor, self);
 
         function is_tail(node, parent) {
-            if (parent instanceof AST_Binary) {
-                return parent.right === node || parent.right.is_constant_expression(scope);
+            if (parent instanceof AST_Binary) switch (node) {
+              case parent.left:
+                return parent.right.is_constant_expression(scope);
+              case parent.right:
+                return true;
+              default:
+                return false;
             }
-            if (parent instanceof AST_Conditional) {
-                return parent.condition !== node
-                    || parent.consequent.is_constant_expression(scope)
+            if (parent instanceof AST_Conditional) switch (node) {
+              case parent.condition:
+                return parent.consequent.is_constant_expression(scope)
                     && parent.alternative.is_constant_expression(scope);
+              case parent.consequent:
+              case parent.alternative:
+                return true;
+              default:
+                return false;
             }
             if (parent instanceof AST_Sequence) {
                 var exprs = parent.expressions;
@@ -12723,7 +12733,7 @@ Compressor.prototype.compress = function(node) {
                 }
                 return true;
             }
-            if (parent instanceof AST_UnaryPrefix) return true;
+            return parent instanceof AST_UnaryPrefix;
         }
 
         function is_tail_block(stat, parent) {

test/compress/dead-code.js

@@ -1705,3 +1705,28 @@ issue_5506: {
         "bar",
     ]
 }
+
+issue_5641: {
+    options = {
+        collapse_vars: true,
+        conditionals: true,
+        dead_code: true,
+    }
+    input: {
+        function f(a) {
+            if (a || b) {
+                var b = "PASS", c = b && console.log(b);
+            } else
+                var d = a || b;
+        }
+        f(42);
+    }
+    expect: {
+        function f(a) {
+            var b, c, d;
+            (a || b) && (b = "PASS") && console.log(b);
+        }
+        f(42);
+    }
+    expect_stdout: "PASS"
+}