[ARM] MVE vector for 64bit types

We need to make sure that we are sensibly dealing with vectors of types v2i64
and v2f64, even if most of the time we cannot generate native operations for
them. This mostly adds a lot of testing, plus fixes up a couple of the issues
found. And, or and xor can be legal for v2i64, and shifts combining needs a
slight fixup.

Differential Revision: https://reviews.llvm.org/D64316

llvm-svn: 366106