[llvm] [RISCV] Combine (addi (addi)) and add post riscv-opt-w-instrs machine-combiner (PR #171165)

[Piotr Fusik via llvm-commits]

pfusik wrote:

This is an alternative to #157416 that also addresses pattern 2.
I'm unsure if running machine-combiner for the second time is the best solution, but I incidentally found that it also enables some SHXADD combines.

https://github.com/llvm/llvm-project/pull/171165