intel/compiler: Lower ffma on Gen4 and Gen5
MR !98 (closed) caused a bunch of failures on older Intel GPUs. At first I thought it was because the lowering pass happened after all of the ffma lowering. The actual problem was that the Intel driver never asked for any ffma lower. Even later versions of !98 (closed) that tried to detect the lowering and avoid generating the ffma failed for this reason.