[llvm] [DA] Remove base pointers from subscripts (NFCI) (PR #157083)

[Ryotaro Kasuga via llvm-commits]

kasuga-fj wrote:

I'll go ahead and land this now, as it affects other patches related to overflow handling. If tests are needed, I'll add them separately.

https://github.com/llvm/llvm-project/pull/157083