[RISCV] Backport db6bee to 16.x release branch

[preames]

This change fixes a recent and high impact regression. The flawed logic was introduced in Dec 22, by change 844430bcc377.

This was recently exposed in a stage2 build of llvm-tablegen when we switched from using llvm::Optional to std::optional. The stars aligned in just the wrong way, and we started actively miscompiling idiomatic optional usage. std::optional<uint32_t> appears to use the top 32 bits of the word on RV64 for its tag.

[preames]

/cherry-pick db6bee5

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

[llvmbot]

Failed to cherry-pick: db6bee5

https://github.com/llvm/llvm-project/actions/runs/4127904694

Please manually backport the fix and push it to your github fork. Once this is done, please add a comment like this:

/branch <user>/<repo>/<branch>

[asb]

/cherry-pick 604cc68 db6bee5

[llvmbot]

/branch llvm/llvm-project-release-prs/issue60608