i965/fs: demote_pull_constants() did not take into account double types

i965/fs: demote_pull_constants() did not take into account double types

The constants could be double, and it was allocating size for float types
for the destination register of varying pull constant loads.

Then the fs_visitor::validate() will complain.

Signed-off-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>