[PATCHSET][ARM64] Implement big endian NEON

[Tim Northover]

Hi James,

> You were quite right - moving the bitcast logic into the .td file allowed a
> trivial patch to take the place of all the ISelLowering changes. Attached
> are the last three patches for LLVM for big endian that I have - they're now
> fairly trivial (but the first one looks large because of testcases).

I think these 3 look good to me.

Thanks for working on them!

Tim.