llvmpipe: clean up shader pre/postamble, try to catch more early-z

llvmpipe: clean up shader pre/postamble, try to catch more early-z

Specifically, can do early-depth-test even when alpahtest or
kill-pixel are active, providing we defer the actual z write until the
final mask is avaialable.

Improves demos/fire.c especially in the case where you get close to
the trees.