[llvm] [RISCV] Convert LWU to LW if possible in RISCVOptWInstrs (PR #144703)

[Alex Bradbury via llvm-commits]

asb wrote:

I'm going to go ahead and land this even though the implementation has changed a bit since the last LGTM:
* The logic change has become trivial since #149710 and the intended approach was flagged in that PR
* The implementation approach hasn't changed meaningfully since the previous review (just in a different function in the same pass)
* There has long been agreement on the approach.

https://github.com/llvm/llvm-project/pull/144703