[llvm] [TailRecElim] Introduce support for shift accumulator optimization (PR #181331)

[Federico Bruzzone via llvm-commits]

FedericoBruzzone wrote:

> @FedericoBruzzone Why was this closed?

I didn't want it to clutter up the PRs section because I thought nobody was interested in it anymore. I'll reopen it if that's not the case :D 

https://github.com/llvm/llvm-project/pull/181331