GBE: disable mad for some cases.

GBE: disable mad for some cases.

One case is one operand is an imm value. Then it turns to
save one instruction but add an extra LOADI instruction.
We don't need to bother to use mad in this case.
And considering when we optimize the simd16 under simd8
mode, we

The other case is under simd16 mode. As mad is a 3-src instruction,
which only support simd8, it will convert one mad(16) instruction to
two mad(8) instructions. Then we don't need to use mad.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Reviewed-by: "Yang, Rong R" <rong.r.yang@intel.com>