[llvm] [TableGen][RISCV] Use getAllDerivedDefinitionsIfDefined in RISCVTargetDefEmitter (PR #91941)

[Alex Bradbury via llvm-commits]

asb wrote:

> I'm not familiar with RISCVTargetDefEmitter.cpp, but makes sense to me. I presume this is so we can test extensions/profiles/CPUs independently?

Yes, writing a standalone test is what raised this issue for me. The precise testing scenario is related to what we were discussing offline about zero-length arrays when there are no experimental extensions or (when I add it) experimental profiles and a bit fiddly to explain here. It's possible the precise tests I was going to do don't make sense in the end, but I'd suggest this change probably still makes sense in isolation.

https://github.com/llvm/llvm-project/pull/91941