Eliminate unnecessary inverse masks.

Eliminate unnecessary inverse masks.

~mask[i % size] is the same as mask[~i % size] because it's just an
expansion of i, so there's no need for invMask[] to store the former.

Change-Id: I754732a2c4978281d35037941ecb74448ab78a8e
Reviewed-on: https://swiftshader-review.googlesource.com/3258
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>