Fix potential buffer over-read in highbd d117 predictor Neon

Fix potential buffer over-read in highbd d117 predictor Neon

The load of `left[bs]` in the standard bitdepth d117 Neon implementation
triggered an address-sanitizer failure.

The highbd equivalent does not appear to trigger any asan failures when
running the VP9/ExternalFrameBufferMD5Test or
VP9/TestVectorTest.MD5Match tests, but for consistency with the standard
bitdepth implementation we adjust it to avoid the over-read.

Performance is roughly identical, with a 0.8% performance improvement on
average over the previous optimised code.

Change-Id: I05dc4d43f244f4915c0ccc52cc0af999bbacb018