[NFCI][AArch64] Use DAG Helper Functions

This commit is a cleanup of a few things in the SelectionDAG
implementation:
- to have more places use `splitInt128`, rather than duplicating that
  functionality when they are turning a 128-bit value into two 64-bit
  values,
- to refactor `splitInt128` to use EXTRACT_ELEMENT which is for
  splitting wide integers in two (rather than the equivalent with shifts
  and truncs), and
- to use the concise helpers for INSERT_/EXTRACT_SUBREG in more places.