[regexp] Avoid unneeded accesses to lastIndex

test/mjsunit/regress/regress-2438.js

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 function testSideEffects(subject, re) {
   var counter = 0;
+  var expected_counter = 0;
+  const accesses_lastindex = (re.global || re.sticky);
   var side_effect_object = { valueOf: function() { return counter++; } };
   re.lastIndex = side_effect_object;
   re.exec(subject);
-  assertEquals(1, counter);
+
+  if (accesses_lastindex) expected_counter++;
+  assertEquals(expected_counter, counter);
 
   re.lastIndex = side_effect_object;
   re.test(subject);
-  assertEquals(2, counter);
+
+  if (accesses_lastindex) expected_counter++;
+  assertEquals(expected_counter, counter);
 }
 
 testSideEffects("zzzz", /a/);
 testSideEffects("zzzz", /a/g);
 testSideEffects("xaxa", /a/);
 testSideEffects("xaxa", /a/g);