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>