intel/fs: Use the ATTR file for FS inputs

This replaces the special magic opcodes which implicitly read inputs
with explicit use of the ATTR file.

v2 (Jason Ekstrand):
 - Break into multiple patches
 - Change the units of the FS ATTR to be in logical scalars

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/compiler/brw_fs.h

@@ -276,7 +276,7 @@ public:
 
    fs_reg get_timestamp(const brw::fs_builder &bld);
 
-   struct brw_reg interp_reg(int location, int channel);
+   fs_reg interp_reg(int location, int channel);
 
    int implied_mrf_writes(fs_inst *inst) const;