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

[Rahul Joshi via llvm-commits]

jurahul wrote:

How widely used? I'd say moderately.

Within LLVM repo: ~600 instances (did git grep "isa<.*, .*>" | wc -l). Within some out-of-tree repos: https://github.com/triton-lang/triton : 88, https://github.com/openxla/xla.git: 30, https://github.com/tensorflow/tensorflow: 200.

But I think its widely applicable. (using git grep "isa<.*>(.*) || isa<.*>(.*)" | wc -l) 1000 in LLVM repo.

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