[AArch64] Scale offsets by the size of the memory operation. NFC.

The immediate in the load/store should be scaled by the size of the memory
operation, not the size of the register being loaded/stored.  This change gets
us one step closer to forming LDPSW instructions.  This change also enables
pre- and post-indexing for halfword and byte loads and stores.

llvm-svn: 248804