AArch64: refactor vector list creation to be more uniform

Instructions taking a vector list (e.g. "ld2 {v0.2d, v1.d2}, [x0]") need a
special register-class to deal with the constraints, and C++ code to support
selection. However, that C++ code can be made reasonably uniform to simplify
the selection process. Hence this patch.

No functionality change, so no tests.