i965: only try print GLSL IR once when using INTEL_DEBUG to dump ir

i965: only try print GLSL IR once when using INTEL_DEBUG to dump ir

Since we started releasing GLSL IR after linking the only time we can
print GLSL IR is during linking. When regenerating variants only NIR
will be available.

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>