nir/opcodes: Add new 'umul_low' and 'imadsh_mix16' opcodes

nir/opcodes: Add new 'umul_low' and 'imadsh_mix16' opcodes

'umul_low' is the low 32-bits of unsigned integer multiply. It maps
directly to ir3's MULL_U.

'imadsh_mix16' is multiply add with shift and mix, an ir3 specific
instruction that maps directly to ir3's IMADSH_M16.

Both are necessary for the lowering of integer multiplication on
Freedreno, which will be introduced later in this series.

Reviewed-by: Eric Anholt <eric@anholt.net>