nir/lower_clip: make the pass compatible with Vulkan semantics

nir/lower_clip: make the pass compatible with Vulkan semantics

Vulkan allows fragment shaders to read gl_ClipDistance[], in which
case the SPIR-V compiler inserts a single compact array variable for
VARYING_SLOW_CLIP_DIST0 and the lowering should not try to inject
its own variables, but instead work in terms of the existing one.
Vulkan drivers are expected to call this with use_clipdist_array set
to true to be consistent with this setup.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6022>