GBE: optimize CMP instruction encoding.

GBE: optimize CMP instruction encoding.

This patch fixes the following two things.
1. Use a temporary register as dst register for the CMP
instruction in the middle of a block.
2. fix the switch flag for the CMP instruction at the begining
of each block. As the compact instruction handling will handle
the cmp instruction directly, and will ignore the switch
flag which is incorrect.

This patch could get about 2-3% performance gain for luxmark.

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