nir: Add intrinsics for the line width

nir: Add intrinsics for the line width

The first intrinsic is intended to expose the value set by glLineWidth
to shaders internally. The second intrinsic exposes the value actually
sent to the hardware. This may be wider than the first one in order to
implement anti-aliasing. These will be used in later patches to
implement a line smoothing lowering pass.

v2: Add a second intrinsic for the expanded line width for
    anti-aliasing.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5624>