[llvm] [RISCV][LSR] Account for temporary register for base addition (PR #92296)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

My level of confidence in LSR changes isn't overly high, as I've found it difficult to modify without having intended consequences. But LGTM.

https://github.com/llvm/llvm-project/pull/92296