[llvm] [GlobalISel] Document minimum legality requirements for G_IMPLICIT_DEF. (PR #117609)

[Amara Emerson via llvm-commits]

aemerson wrote:

> I disagree with that change. It put's unnecessary burden on implementations with limited gain.

What burden are you talking about? How does this change anything that isn't already the case? I'm not sure how else to restate what I've already said.

https://github.com/llvm/llvm-project/pull/117609