nir: partially revert c2acf97fcc9b32e

nir: partially revert c2acf97fcc9b32e

c2acf97fcc9b32e changed the use of double_inputs_read to be
inconsitent with its previous meaning. Here we re-enable the
gather info code that was removed as the modified code from
c2acf97fcc9b32e now uses the double_inputs member rather than
double_inputs_read.

This change allows us to use double_inputs_read with gallium
drivers without impacting double_inputs which is used by i965.

We also make use of the compiler option vs_inputs_dual_locations
to allow for the difference in behaviour between drivers that handle
vs inputs as taking up two locations for doubles, versus those that
treat them as taking a single location.

Reviewed-by: Karol Herbst <kherbst@redhat.com>