[llvm] [LLVM] Change Intrinsic::ID to encode target and intrinsic index (PR #113576)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Have you checked whether this does help incremental builds in practice?

I checked and it does seem to help. Adding a new AMDGPU intrinsic did not cause any other target's `.cpp` files to be rebuilt. (But TBH I'm still not a huge fan of this patch, just because it seems like extra complexity that we don't really need.)

https://github.com/llvm/llvm-project/pull/113576