[llvm] [RISCV] Use addi rather than addiw for immediates materialised by lui+addi(w) pairs when possible (PR #141663)

[Alex Bradbury via llvm-commits]

asb wrote:

I've edited the PR description and written up the extra testing + analysis there. It turns out this is actually results in a meaningful reduction of dynamic instcount for 519.lbm_r (~4%).

This is now ready for re-review.

https://github.com/llvm/llvm-project/pull/141663