[llvm] [RISCV] Enable load clustering by default (PR #73789)

[Alex Bradbury via llvm-commits]

asb wrote:

For reference, #74600 shows the additional diff if you then enable the selectiondag scheduler. Lots of cases where there's just noise (e.g. different register choice), but a few I want to look at more closely as ideally we'd get by only enabling load clustering in the MachineScheduler I think.

https://github.com/llvm/llvm-project/pull/73789