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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

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

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