i965: Don't set NirOptions for stages that will use the vec4 backend.

i965: Don't set NirOptions for stages that will use the vec4 backend.

We've started using NirOptions != NULL to mean "we're using NIR for this
stage."  However, when INTEL_USE_NIR=1, we set it for a bunch of stages
that still use the vec4 backend, and thus definitely aren't using NIR.

For example, if INTEL_USE_NIR=1 we disable the GLSL IR cubemap
normalization pass, even for vertex shaders and geometry shaders.  This
is wrong, but breaks a very uncommon case.

When I started deleting GLSL IR for stages where we claimed to be using
NIR, this bug quickly became apparent.

For now, only set it for fragment shaders, and vertex shaders if
brw->scalar_vs is set.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>