llvmpipe: fix early depth test / late depth write stencil issues

llvmpipe: fix early depth test / late depth write stencil issues

We actually did early depth/stencil test and late depth/stencil write even
when the shader could kill the fragment (alpha test or discard). Since it
matters for the new stencil value if the fragment is killed by depth/stencil
test or by the shader (in which case it will not reach the depth/stencil
test) this simply cannot work (we also would possibly skip writing the new
stencil value due to mask checks but this is a secondary issue).
So use late depth test / late depth write instead in this case.
(No piglit changes as it doesn't seem to hit such bogus early depth test
/ late depth write path.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>