[regexp] Again support regexp execution without side effects

This is a quirk needed for the regexp fuzzer, which passes its own
custom RegExpMatchInfo object to RegExpImpl::Exec and expects execution
without side effects.

Bug: chromium:934621
Change-Id: I90286fda06593d7c574d8d4629481ebad2fa5b1d
Reviewed-on: https://chromium-review.googlesource.com/c/1485833Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59839}

src/regexp/jsregexp.cc

@@ -564,8 +564,13 @@ Handle<RegExpMatchInfo> RegExpImpl::SetLastMatchInfo(
   result->SetNumberOfCaptureRegisters(capture_register_count);
 
   if (*result != *last_match_info) {
-    DCHECK_EQ(*last_match_info, *isolate->regexp_last_match_info());
-    isolate->native_context()->set_regexp_last_match_info(*result);
+    if (*last_match_info == *isolate->regexp_last_match_info()) {
+      // This inner condition is only needed for special situations like the
+      // regexp fuzzer, where we pass our own custom RegExpMatchInfo to
+      // RegExpImpl::Exec; there actually want to bypass the Isolate's match
+      // info and execute the regexp without side effects.
+      isolate->native_context()->set_regexp_last_match_info(*result);
+    }
   }
 
   DisallowHeapAllocation no_allocation;