i965: Use hash tables for brw_fs_vector_splitting().

i965: Use hash tables for brw_fs_vector_splitting().

Cuts compile/link time of the fragment shader in #91857 by 25%
(21.64 -> 16.28).

v2: Drop unnecessary _mesa_hash_table_destroy call, and use
    refs.ht->entries == 0 rather than ad-hoc checking (suggested by
    Timothy Arceri).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=91857
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>
Reviewed-by: Timothy Arceri <t_arceri@yahoo.com.au>
Tested-by: Tapani Pälli <tapani.palli@intel.com>