backend: refine load/store merging algorithm

Now it works for sequence: load(0), load(1), load(2)
but it cant work for load(2), load(0), load(1). because
it compared the last merged load and the new one not all
the loads

for  sequence: load(0), load(1), load(2). the load(0) is the
start, can find that load(1) is successor without space, so
put it to a merge fifo. then the start is moving to the top
of fifo load(1), and compared with load(2). Also load(2) can be merged

for load(2), load(0), load(1). load(2) cant be merged with
load(0) for a space between them. So skip load(0) and mov to next
load(1).And this load(1) can be merged. But it never go back merge
load(0)

Now change the algorithm.
(1) find all loads maybe merged arround the start by the distance to
    the start. the distance is depended on data type, for 32bit data, the
    distance is 4. Put them in a list

(2) sort the list by the distance from the start.

(3) search the continuous sequence including the start to merge

V2: (1)refine the sort and compare algoritm. First find all the IO
       in small offset compared to start. Then call std:sort
    (2)check the number of candidate IO to be favorable to performance
       for most cases there is no chance to merge IO

Signed-off-by: rander.wang <rander.wang@intel.com>
Reviewed-by: Ruiling Song <ruiling.song@intel.com>

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