ASAN bot caught bug in FastIntegerMap.BasicUsage test.
The end() iterator points past the end of the list, accessing
end()->first is invalid. rbegin() points at the last element of the
list which is what this test intended.

Bug: b/166255370
Change-Id: I06e098143d3de9658088cda6f10761d41b17a520
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2375305Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>