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

[Amara Emerson via llvm-commits]

aemerson wrote:

> You can just an add cache to the legalizer instead of introducing harsher rules. As a fall-back IMPLICIT_DEF is still available.

Caches are not free either. I'll change the wording in this PR but this is documenting existing de facto convention for good reason. Your feedback has been noted.

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