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/+/2842347Reviewed-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>