[AArch64] Add indexed check to splitStores. NFC.

Add explicit check for unhandled cases in preparation for delaying
splitStores to post-legalization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305471 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed