angle::BitMask(n) implements the common pattern of angle::Bit(n)-1.

Bug: angleproject:6048
Change-Id: Icd56ef1504804add59d0804a7249b3035c96f9c2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2984099
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>