[llvm] [RISCV][GISel] Emit G_CONSTANT 0 as a copy from X0. (PR #67202)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland approved this pull request.

LGTM with the precommit. I understand the copyprop was disabled to show the diff on the pre-commit. Thanks for adding that.

https://github.com/llvm/llvm-project/pull/67202