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

[Craig Topper via llvm-commits]

topperc wrote:

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

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