[ARM] Fix incorrect handling of big-endian vmov.i64

Currently when the target is big-endian vmov.i64 reverses the order of the two
words of the vector. This is correct only when the underlying element type is
32-bit, as actually what it should be doing is considering it a vector of the
underlying type and reversing the elements of that.

Differential Revision: https://reviews.llvm.org/D76515