i965: Replace a should-never-happen fallback with asserts where it matters.

i965: Replace a should-never-happen fallback with asserts where it matters.

We only allow 16 vec4s of attributes in our GLSL/ARB_vp programs, and
1 more element will get used for gl_VertexID/gl_InstanceID.  So it
should never have been possible to hit this fallback, unless there was
another bug.  If you do hit this, you're probably using gl_VertexID
and falling back to swrast won't work for you anyway.

This also updates the limits for gen6+.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>