[AArch64][InstructionSelector] Teach the selector how to handle vector OR.

This only adds the support for 64-bit vector OR. Adding more sizes is
not difficult, but it requires a bigger refactoring because ORs work on
any size, not necessarly the ones that match the width of the register
width. Right now, this is not expressed in the legalization, so don't
bother pushing the refactoring yet.

llvm-svn: 283831