There appears to have been some confusion over `uint_mask` during the
initial reviews of the subgroup tests. Add a comment to clarify the
purpose of the mask.
Use a 32-bit all-ones mask (`0xffffffff`) instead of a 64-bit all-ones
mask to further clarify the intent, that is, don't delay discarding
the upper 32 bits until the assignment but mask them out immediately.
Fixes https://github.com/KhronosGroup/OpenCL-CTS/issues/1203
Signed-off-by: Sven van Haastregt <sven.vanhaastregt@arm.com>
48003e9d32