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

[Sam Elliott via llvm-commits]

lenary wrote:

FYI @saloni-shinde-Q is a developer who recently joined my team and is continuing to look at the error messages after the last PR she landed. She has been looking at exactly how the near misses are resolved in erroneous examples like `fcvt.s.l a2, ft2`, which sort-of point at needing a better way to choose the nearest miss, especially when the fallback mechanism from #206390 ends up getting hit. That work is a little different to the memory operand work, but it's probably good to be aware of what we're all doing to improve the parser.

It feels like there are plenty of rougher edges to Near Misses, but also to e.g. `validateTargetOperandClass`, so there's lots to do.

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