[llvm] [RISCV] Flatten the ImpliedExts table in RISCVISAInfo.cpp (PR #89975)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

LGTM, thanks again for your work on moving extension definitions to tblgen.

https://github.com/llvm/llvm-project/pull/89975