[ARM] Minor cleanup to CombineBaseUpdate. NFC.

In preparation for a future patch:
- rename isLoad to isLoadOp: the former is confusing, and can be taken
  to refer to the fact that the node is an ISD::LOAD.  (it isn't, yet.)
- change formatting here and there.
- add some comments.
- const-ify bools.

llvm-svn: 229929