Array accesses with unsigned indices can be faster on x86-64 because
we can take advantage of implicit zero-extension of 32-bit integers to
64-bit during pointer arithmetic.

Change-Id: I17d531d9ad05c2d2994f007d5444b2a514a591b8
Reviewed-on: https://swiftshader-review.googlesource.com/8571Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>