i965/fs: Use VARYING_SLOT checks rather than strcmp().

i965/fs: Use VARYING_SLOT checks rather than strcmp().

Comparing the location field is equivalent and more efficient.

We'll also need this when we start using NIR for ARB programs, as our
NIR converter will set the location field correctly, but probably won't
use the GLSL names for these concepts.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>