GBE: use S16 vector to represent bool.

GBE: use S16 vector to represent bool.

The original purpose of using flag or a S16 scalar to represent
a bool data type is to save register usage. But that bring too
much complex to handle it correctly in each possible case. And
the consequent is we have to take too much care about the bool's
handling in many places in the instruction selection stage. We
even never handle all the cases correctly. The hardest part is
that we can't just touch part of the bit in a S16 scalar register.
There is no instruction to support that. So if a bool is from
another BB, or even the bool is from the same BB but there is
a backward JMP and the bool is still a possible livein register,
thus we need to make some instructions to keep the inactive lane's
bit the original value.

I change to use a S16 vector to represent bool type, then all
the complicate cases are gone. And the only big side effect is
that the register consumption. But considering that a real
application will not have many bools active concurrently, this
may not be a big issue.

I measured the performance impact by using luxmark. And only
observed 2%-3% perfomance regression. There are some easy
performance optimization opportunity remains such as reduce
the unecessary MOVs between flag and bool within the same
block. I think this performance regression should be not a
big deal. Especially, this change will make the following if/endif
optimization a little bit easier.

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