glsl: Remove VS output varyings which are optimized out of the FS

glsl: Remove VS output varyings which are optimized out of the FS

Previously when an input varying was optimized out of the
FS we would still retain it as an output of the VS.

We now build a hash of live FS input varyings rather
than looking in the FS symbol table. (The FS symbol table
will still contain the optimized out varyings.)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>