[llvm] [llvm][RISCV] Make X0 register pair legal in pre-ra pass (PR #169164)

[Brandon Wu via llvm-commits]

4vtomat wrote:

> So we have x0 as a store operand pre-ra? I thought it was a copy until the register coalescer pass runs.

I think you're right, it's a copy when we are in this pass

https://github.com/llvm/llvm-project/pull/169164