nir/lower_int_to_float: Make sure the cursor is in the right spot.

nir/lower_int_to_float: Make sure the cursor is in the right spot.

We need to make get it updated after we may have nir_instr_remove()d an
instruction, and when we cross blocks.  This didn't really matter before
because the only builder usage was idiv, which other users of
lower_int_to_float were probably never hitting.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11329>