[llvm] Support replacing `add rd, Zero, Zero` with `c.li rd, 0` (PR #86937)

[Alex Bradbury via llvm-commits]

asb wrote:

> Why should add rd, Zero, Zero be special? If we support this, shouldn't we do it for And, Or, Xor, Sll, Srl, Sra that all produce 0 with x0, x0 arguments?
> 
> Given the number of cases, I don't think we should do this.

I'd say it's more likely that `add rd, zero, zero` is written by a less experienced programmer meaning to have the effect of `li, rd, 0`. But at that point we're picking and choosing which edge cases we handle here which I admit is less attractive vs being able to say "we compress all instructions if possible". I don't feel too strongly about this.

https://github.com/llvm/llvm-project/pull/86937