nir: Convert the NIR instruction insertion API to use cursors.

nir: Convert the NIR instruction insertion API to use cursors.

This patch implements a general nir_instr_insert() function that takes a
nir_cursor for the insertion point.  It then reworks the existing API to
simply be a wrapper around that for compatibility.

This largely involves moving the existing code into a new function.

Suggested by Connor Abbott.

v2: Make the legacy functions static inline in nir.h (requested by
    Connor Abbott).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Acked-by: Connor Abbott <cwabbott0@gmail.com>