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

[Sam Elliott via llvm-commits]

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lenary wrote:

These cannot be a `%*lo(sym)`, which is why I was using `simm12`. I guess we need a `simm12_optional`? Not sure.

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