Improve bounds checking in vp8_diamond_search_sadx4()

In order to know if all 4/8 neighbor points are within the bounds,
4 bounds checking are enough instead of checking 4 bounds for
each points (16/32 checkings). This improvement reduces cost of
vp8_diamond_search_sadx4() by 30%, and gives encoder a 1.5%
performance gain (test options: 1 pass, good, speed=4).

Change-Id: Ie8da29d18a6ecfc9829e74ac02f6fa70e042331a