nir/copy_prop_vars: keep track of components in copy_entry

nir/copy_prop_vars: keep track of components in copy_entry

When a copy_entry is SSA, store not only the nir_ssa_def* for each
component, but also the source component they come from.  At the
moment this is always a match (i.e. 'component[i] == i'), because all
the operations for a copy_entry happen using definitions with the same
size.  This prepares the code for array_derefs of vectors, in which
'component[i] != i'.

Also, extract setting all SSA components into a function of its own.

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