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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

In GitHub style, he commented. He didn't approve in GitHub style.

I am in favour. I don't read that as GitHub style approval.

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