[llvm] [vim] Improve iskeyword for LLVM IR (PR #117905)

[via llvm-commits]

ldrumm wrote:

I'm a big fan of this. Like you say: it's not perfect, but it makes following SSA values so much easier that I think it's definitely worth it despite not covering all complex identifiers.

I'll let someone with more vim chops chime in, but big +1 from me

https://github.com/llvm/llvm-project/pull/117905