[llvm] [IR] Reorder Value fields to put the SubclassID first (PR #75634)

[Fangrui Song via llvm-commits]

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

I've made similar changes to lld/ELF for x86. For `Value` to be beneficial, this means that `getOpcode` and other operations are more important than `getType()`, which seems true.

https://github.com/llvm/llvm-project/pull/75634