AArch64: use explicit MVT::i64 when creating EXTRACT_SUBVECTOR nodes.

All our patterns use MVT::i64, but the ISelLowering nodes were inconsistent in
their choice.

No functional change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223551 91177308-0d34-0410-b5e6-96231b3b80d8