aco: add helpers for splitting stores

aco: add helpers for splitting stores

split_store_data() splits a vector and p_as_uniforms it if needed.

scan_write_mask()/advance_write_mask() are similar to
u_bit_scan_consecutive_range(), but makes it easier to only clear part of
the range and will also give ranges for zero'd bits.

split_buffer_store() is a helper for splitting VMEM/SMEM stores.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4639>