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: b5ac381d8f ("gallivm: add buffer operations to the tgsi->llvm conversion.")
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/gallium/auxiliary/gallivm/lp_bld_tgsi_soa.c

@@ -3585,6 +3585,8 @@ atomic_emit(
    case TGSI_OPCODE_ATOMIMAX:
       op = LLVMAtomicRMWBinOpMax;
       break;
+   case TGSI_OPCODE_ATOMCAS:
+      break;
    default:
       assert(0);
       return;