[llvm] Add late optimization pass for riscv (PR #117060)

[Mikhail R. Gadelha via llvm-commits]

mikhailramalho wrote:

Yesterday I noticed `RISCVInstrInfo::optimizeCondBranch`, which has code to detect `ADDI x0, imm`. Maybe we can use it in the future to handle this case of `bez/bnez zero` simplification. 

Currently, it works only for virtual regs, and it's designed to simplify some cases of blt/bge, but it should be straightforward to change it

https://github.com/llvm/llvm-project/pull/117060