fix corner case in evaluate & unsafe_math (#3756)

fixes #3755

lib/compress.js

@@ -3243,6 +3243,7 @@ merge(Compressor.prototype, {
             }
             if (isNaN(result)) return compressor.find_parent(AST_With) ? this : result;
             if (compressor.option("unsafe_math")
+                && !ignore_side_effects
                 && result
                 && typeof result == "number"
                 && (this.operator == "+" || this.operator == "-")) {

test/compress/evaluate.js

@@ -2174,3 +2174,19 @@ issue_3738: {
     }
     expect_stdout: "Infinity"
 }
+
+issue_3755: {
+    options = {
+        booleans: true,
+        evaluate: true,
+        unsafe: true,
+        unsafe_math: true,
+    }
+    input: {
+        console.log((/4/.exec(1 + (!0 - 5 / "23")) || 0).p);
+    }
+    expect: {
+        console.log((/4/.exec(!0 - 5 / "23" + 1), 0).p);
+    }
+    expect_stdout: "undefined"
+}