i965: Enable vector shrinking in the vec4 backend.

i965: Enable vector shrinking in the vec4 backend.

This manages to make some extra vec operations that would turn into movs
go away.

brw shader-db:
total instructions in shared programs: 3895037 -> 3893221 (-0.05%)
total cycles in shared programs: 113832759 -> 113792154 (-0.04%)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6050>