llvmpipe: fix stencil bug if we have both stencil and depth tests

llvmpipe: fix stencil bug if we have both stencil and depth tests

This is a very well hidden bug found by accident (only the fixed glean
tstencil2 test so far seems to hit it).
We must use new mask with combined s_pass values and orig_mask values
for zpass/zfail stencil ops, otherwise both the sfail op and one of
zpass/zfail op are applied (probably not hit in most tests because
some of the ops tend to be KEEP usually).

Note: this is a candidate for the 9.2 branch.

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