[Hexagon] Flip hexagon-autohvx to be true by default

This will allow other generators of LLVM IR to use the auto-vectorizer
without having to change that flag.

Note: on its own, this patch will disable auto-vectorization on Hexagon
in all cases, regardless of the -fvectorize flag. There is a companion
LLVM patch that together with this one forms an NFC for clang users.

llvm-svn: 345170