[llvm] [RISCV] Add additional fence for amocas when required by recent ABI change (PR #101023)

[Alex Bradbury via llvm-commits]

asb wrote:

> RISCVCodeGenPrepare doesn't run at -O0. Is that a problem?

Good catch, that is indeed a problem. I've pushed a patch that naively just moves this to a minimal separate pass that is guaranteed to run even at O0. What do you think?

https://github.com/llvm/llvm-project/pull/101023