[clang] [llvm] [RISCV][ISel] Ensure 'in X' Constraints prevent X0 (PR #112563)

[Sam Elliott via cfe-commits]

lenary wrote:

Yeah, I went back and looked at that issue, it's helpful to know that it was more than just the register allocator, that it was also copy propagation, which wasn't obvious from the comments. 

The failing tests are not related, but presumably caused by the rebase, but I'll still wait a bit to merge this.

https://github.com/llvm/llvm-project/pull/112563