nir: define behavior of nir_op_bfm and nir_op_u/ibfe according to SM5 spec.

nir: define behavior of nir_op_bfm and nir_op_u/ibfe according to SM5 spec.

That is: the five least significant bits provide the values of
'bits' and 'offset' which is the case for all hardware currently
supported by NIR and using the bfm/bfe instructions.
This patch also changes the lowering of bitfield_insert/extract
using shifts to not use bfm and removes the flag 'lower_bfm'.

Tested-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>