[ARM] Also support v2f64 vld1/vst1.

It was missing from the VLD1/VST1 handling logic, even though the
corresponding instructions exist (same form as v2i64).

In preparation for a future patch.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223832 91177308-0d34-0410-b5e6-96231b3b80d8