[llvm] [RISC-V] Use an optional offset operand instead of zero-offset InstAliases (PR #210901)

[Alexander Richardson via llvm-commits]

arichardson wrote:

Would you like to see a new test showing the difference in diagnostic or is this okay without a test as a "NFCI" commit?

https://github.com/llvm/llvm-project/pull/210901