gallivm: fix atomic compare-and-swap

Not sure how I missed this before, but compswap was hitting an
assert here as it is it's own special case.

Fixes: b5ac381d ("gallivm: add buffer operations to the tgsi->llvm conversion.")
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
(cherry picked from commit 1eda49cc)