nir/spirv: Fix handling of vector component selects via OpAccessChain

nir/spirv: Fix handling of vector component selects via OpAccessChain

When we get to the end of the _vtn_load/store_varaible recursion, we may
have one link left in the deref chain if there is a vector component select
on the end.  In this case, we need to truncate the deref chain early so
that, when we make the copy for the load, we don't get the extra deref.
The final deref will be handled by the vector extract/insert that comes
later.