[NFC][AArch64] Use 'i' to encode the offset form of load/store.

STG, STZG, ST2G, STZ2G are the exceptions to append 'Offset' to name the
offset format of load/store instructions. All other load/store
instructions use 'i' as the appendix. If there is no special reason to
do so, we should make the naming consistent.

Differential Revision: https://reviews.llvm.org/D141819