gallivm: simplify geometry shader mask handling a bit

gallivm: simplify geometry shader mask handling a bit

Instead of reducing masks to 0/1 simply use the mask directly as -1.
Also use some signed comparison instead of unsigned (as far as I understand
these values have to be (very) small and signed means llvm doesn't have to
apply additional logic to do the unsigned comparisons the cpu can't do).
Saves a couple of instructions in some test geometry shader here.

v2: that was a bit to much optimization, don't skip combining the masks...

Reviewed-by: Zack Rusin <zackr@vmware.com>