[llvm] [RISCV] GISel custom lowering for addiw (PR #121587)

[Luke Quinn via llvm-commits]

lquinn2015 wrote:

@topperc push a second commit to covers G_ADD/G_SUB and generates addw/addiw/subw /negw instructions like expected. There are no subiw instructions to generate for RISCV to my understanding. Changes the DestAdd ->DestALU because it makes more sense now

https://github.com/llvm/llvm-project/pull/121587