i965/vec4_nir: Use partial SSA form rather than full non-SSA

i965/vec4_nir: Use partial SSA form rather than full non-SSA

We made this switch in the FS backend some time ago and it seems to make a
number of things a bit easier.  In particular, supporting SSA values takes
very little work in the backend and allows us to take advantage of the
majority of the SSA information even after we've gotten rid of Phi nodes.

Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>