Fix undefined behaviour sanitiser errors (avx2 + LHCbMath/Similarity)

I ran locally with the undefined behaviour sanitiser in an avx2-enabled build and saw a couple of errors, one alignment issue:

../Kernel/LHCbMath/src/Similarity.cpp:94:12: runtime error: load of misaligned address 0x7ffcee0396e8 for type '__m256d', which requires 32 byte alignment

and one out-of-range access

@sponce can you take a look? I'm not that familiar with Vc, maybe there is a better/nicer way to make explicit that these are unaligned loads?

cc: @jonrob