nv50/ir: add preliminary support for SHLADD

nv50/ir: add preliminary support for SHLADD

This instruction is available since SM20 (Fermi) and allow to do
(a << b) + c in one shot. In some situations, IMAD should be
replaced by SHLADD when b is a power of 2, and ADD+SHL should be
replaced by SHLADD as well.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>