AArch64: fix and enable sparse- and packed-switch.

Changed implementation of sparse- and packed-switch to use w and x
registers appropriately. Also added a couple of utilities to obtain a
w/s register corresponding to a given x/d register and viceversa.

Change-Id: I485a110f6e91b09227d9e2a0b8b14224a10bca90
3 files changed