backend: refine load store optimization
backend: refine load store optimization

this fix basic test in conformance tests failed for vec8 of char because
of overflow. And it fix many test items failed in opencv because of offset error

(1)modify the size of searchInsnArray to 32, it is the max size for char
   And add check for overflow if too many insn
(2)Make sure the start insn is the first insn of searched array
   because if it is not the first, the offset maybe invalid. And
   it is complex to modify offset without error

V2: refine search index, using J not I
V3: remove (2), now add offset to the pointer of start
    pass OpenCV, conformance basic and compiler tests, utests
V4: check pointer type, if 64bit, modify it by 64, or 32
V5: refine findSafeInstruction() and variable naming in
    findConsecutiveAccess().

Signed-off-by: rander.wang <5299db4a9bf784920a3e558099f6736bcd8939e6@intel.com>
Signed-off-by: Ruiling Song <286a174c1339c6eff91abc41c3a1012c95030c95@intel.com>
Reviewed-by: Yang Rong <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet