[llvm] [NFC][LLVM][IR] Adopt vadiadic `isa<>` (PR #137001)

[Rahul Joshi via llvm-commits]

jurahul wrote:

What would be the rationale? It's been around for ~5 years (I had added it when I was doing MLIR work) and I suspect has uses out of tree as well. It's a convenient shorthand for a common code pattern.

I'll do some grepping and report back how widely it's used (both upstream and also if I find any out of tree uses from a quick search).

https://github.com/llvm/llvm-project/pull/137001