nir/copy_prop_vars: handle load/store of vector elements

nir/copy_prop_vars: handle load/store of vector elements

When direct array deref is used on a vector type (for loads and
stores), copy_prop_vars is now smart to propagate values it knows
about.

Given a 'vec4 v', storing to v[3] will update the copy entry for v and
it is equivalent to a write to v.w.  Loading from v[1] will try first
to see if there's a known value for v.y -- and drop the load in that
case.

The copy entries still always refer to the entire vectors, so the
operations happen on the parent deref (the 'vector') and the values
are fixed accordingly.

It might be the case now that certain entries have not only different
SSA defs in each element but also those come from different components
than they are set to, because stores to individual elements always
come from a SSA definition with a single component.

Tests related to these cases are now enabled.

v2: Instead of asserting on invalid indices, "load" an undef and
    remove the store.  (Jason)

v3: Merge code path for the cases of is_array_deref_of_vector into the
    regular code path.  Add a base_index parameter to
    value_set_from_value.  (code changes by Jason)

v4: Removed the get_entry_for_deref helper, now being used only once.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>