Fix ScanReverse on posix/32-bit. The use of the __builtin_clz intrinsic and the associated math were incorrect. Bug: angleproject:5736 Change-Id: I9627c7fc179c0e1bffeecaee39f7a88d9c62d079 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2845232 Commit-Queue: Yuly Novikov <ynovikov@chromium.org> Reviewed-by: Yuly Novikov <ynovikov@chromium.org>