Borrow the trick from memset and memmove and use the scale/index/base addressing
to avoid branches.
If a mismatch is found, the routine has to calculate the difference. Make sure
there is always up to 8 bytes to inspect. This replaces the previous loop which
would operate over up to 16 bytes with an unrolled list of 8 tests.
Speed varies a lot, but this is a net win over the previous routine with probably
a lot more to gain.
Validated with glibc test suite.
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