nir/copy_prop_vars: handle indirect vector elements

nir/copy_prop_vars: handle indirect vector elements

Differently than the direct case, the indirect array derefs of vector
are handled like regular derefs, with the exception that we ignore any
vector entry that has SSA values when performing a load.  Such SSA
values don't help loading of the indirect unless we emit an if-ladder.

Copy_derefs are supported for indirects.

Also enable two tests that now pass.

v2: Remove unnecessary temporaries.  Be clearer when identifying the
    case where copy_entry doesn't help when we are dealing with an
    indirect array_deref (of a vector).  (Jason)

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