GBE: refine the gen program strategy.

GBE: refine the gen program strategy.

The limitRegisterPressure only affects the MAD pattern matching
which could not bring noticeable difference here. I change it to always
be false. And add the reserved registers for spill to the strategy
structure. Thus we can try to build a program as the following
strategy:

1. SIMD16 without spilling
2. SIMD16 with 10 spilling registers and with a default spilling threshold
   value 16. When need to spill more than 16 registers, we fall back to next
   method.
3. SIMD8 without spilling
4. SIMD8 with 8 spilling registers.

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>
Reviewed-by: "Song, Ruiling" <ruiling.song@intel.com>