[ARM] Factor out base-updating VLD/VST combiner function. NFC.

Move the combiner-state check into another function, add a few
small comments, and use a more general type in a cast<>.

In preparation for a future patch.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223834 91177308-0d34-0410-b5e6-96231b3b80d8