llvmpipe: fix multisample lines again

llvmpipe: fix multisample lines again

This does a little bit better than what we did in 2c0a078fdb4
("llvmpipe: fix multisample lines."), where parts of the diamond-exit
rule stuff was bypassed. But we should actually bypass *all* of the
diamond-exit rule stuff here instead.

The reason is that multisampled lines have a completely differently
specified set of rasterization rules, as per the OpenGL 4.6 core spec,
section 14.5.4 ("Line Multisample Rasterization").

So let's give multisampled lines their own geometry-generation codepath
instead.

This fixes the following dEQP tests:
- dEQP-GLES3.functional.rasterization.fbo.rbo_multisample_4.primitives.lines
- dEQP-GLES3.functional.rasterization.fbo.rbo_multisample_max.primitives.lines

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11183>