Add first() and last() to BitSetArray. These helper functions return the bounds of the 1 bits in the array. Also required implementing ScanReverse the same way we implemented ScanForward. Bug: angleproject:5736 Change-Id: Ied945c57cd85ca7bc91dcc7a1168a74b3a59fce4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2842347 Reviewed-by: Mohan Maiya <m.maiya@samsung.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>