util: Add util_widen_mask function.

util: Add util_widen_mask function.

Widens the given bit mask by a multiplier, meaning that it will
replicate each bit by that amount.
For example: 0b101 widened by 2 will become: 0b110011

This is typically used in shader I/O to transform a 64-bit
writemask to a 32-bit writemask.

Moving this function here because it is used in multiple places.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/14005>