replace x >= 0 && x < size by (unsigned)x < size
By converting signed coordinate to unsigned number, negative values became
very large positive ones, larger than every positive signed number and
therefore also bigger than the map size. As a result check against size
also implicitly checks if coordinate is negative.
Compiler cannot do this transformation automatically because it doesn't
know that map dimensions are always positive.
The change shrinks isInTheMap from 19 instructions to 11 on x86.
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