i965: Do NIR shader cloning in the caller.

i965: Do NIR shader cloning in the caller.

This moves nir_shader_clone() to the driver-specific compile function,
rather than the shared src/intel/compiler code.  This allows i965 to do
key-specific passes before calling brw_compile_*.  Vulkan should not
need this cloning as it doesn't compile multiple variants.

We do need to continue cloning in the compute shader code because we
lower various things in NIR based on the SIMD width.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>