Improve a few string operations with AVX2

AVX2 brings the new PMOVZXBW instruction that extends from one 128-bit
SSE register to an 256-bit AVX register. With that, the main decoding
code is just two instructions (the loop requires a couple more to
maintain the offset counter and do the end-of-loop check).

This buys us another 4% performance improvement in the fromLatin1 code,
calculated on top of the VEX-encoded SSE2 code (which is already a little
better than plain SSE2).

Change-Id: I675fa24de4fa97683b662f19d146047251f77359
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>