fix corner case in dead_code

fixes #3552

lib/compress.js

@@ -6433,6 +6433,7 @@ merge(Compressor.prototype, {
     OPT(AST_Assign, function(self, compressor) {
         if (compressor.option("dead_code")) {
             if (self.left instanceof AST_PropAccess) {
+                if (self.operator == "=") {
                     var exp = self.left.expression;
                     if (exp instanceof AST_Lambda
                         || !compressor.has_directive("use strict")
@@ -6443,6 +6444,7 @@ merge(Compressor.prototype, {
                             self.right
                         ]).optimize(compressor);
                     }
+                }
             } else if (self.left instanceof AST_SymbolRef) {
                 var def = self.left.definition();
                 if (def.scope === compressor.find_parent(AST_Lambda)) {

test/compress/dead-code.js

@@ -1042,3 +1042,25 @@ function_assign: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3552: {
+    options = {
+        dead_code: true,
+        pure_getters: "strict",
+    }
+    input: {
+        var a = "PASS";
+        (function() {
+            (1..p += 42) && (a = "FAIL");
+        })();
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS";
+        (function() {
+            (1..p += 42) && (a = "FAIL");
+        })();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+}