llvmpipe: disable simple_shader optimization

llvmpipe: disable simple_shader optimization

This optimization disabled mask checks if the shader is simple enough.
While this should work correctly, the problem is that it can hide real issues
because shaders in practice are usually complex enough (8 instructions or 1
texture is already enough) so this doesn't get used, whereas dumbed-down
tests which should hit all the same code paths suddenly do something quite
different. This was the reason that bug 41787 could not be easily tracked as
stencil test not working correctly (piglit would in fact have failed some
tests without that optimization).
So disable it for now, it's unclear if it's much of a win in any case.

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