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

[Sam Elliott via cfe-commits]

lenary wrote:

I spent some time trying to write a test that should fail before, but doesn't after, but it doesn't happen - I presume because x0 is reasonably dealt with by the allocator. This maybe suggests the patch isn't needed? I'm not sure why we do this for GPRs then.

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