nir/builder: Use nir_after_instr to advance the cursor

nir/builder: Use nir_after_instr to advance the cursor

This *should* ensure that the cursor gets properly advanced in all cases.
We had a problem before where, if the cursor was created using
nir_after_cf_node on a non-block cf_node, that would call nir_before_block
on the block following the cf node.  Instructions would then get inserted
in backwards order at the top of the block which is not at all what you
would expect from nir_after_cf_node.  By just resetting to after_instr, we
avoid all these problems.