i965: check for dual slot attributes on any gen
i965: check for dual slot attributes on any gen

Those not supporting 64 bit input vertex attributes will have the
dual_slot value as false.

Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler