GBE: refactor double support.

GBE: refactor double support.

There are two major issues in double support:
1. Doesn't work at SIMD16 mode.
2. The incorrect usage of vectors. We only need to allocate
those temporary register to contiguous registers.

If you look at the previous implementation of
READ_FLOAT64/WRITE_FLOAT64 in gen_encoder.cpp. You can easily
find it contains many duplicate code and considering the SIMD16
code path never work correctly, it's so difficult to work based
on that code. So I choose to refactor those two major functions.
And refine other parts in the instruction selection stage to fix
the above two major problem with a cleaner code.

Now, it works well on both SIMD16/SIMD8 mode.
Another minor improvement is for the READ_FLOAT64 on SIMD8 mode,
this patch saves one time of send instruction to read all the
8 double data into registers.

Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>
Reviewed-by: Homer Hsing <homer.xing@intel.com>