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

[Alexander Richardson via llvm-commits]

arichardson wrote:

> LGTM, but I would prefer we revert the CompressPat changes if the emitter is not complaining (it is probably complaining 😦 )

looks like it actually does work and my search+replace was too aggressive. dropped.

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