[globalisel][legalizer] Attempt to write down the minimal legalization rules

[globalisel][legalizer] Attempt to write down the minimal legalization rules

Summary:
There aren't very many requirements on the legalization rules but we should
document them.

Reviewers: aditya_nandakumar, volkan, bogner, paquette, aemerson, rovka, arsenm, Petar.Avramovic

Subscribers: wdng, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62423

# Conflicts:
#	llvm/docs/GlobalISel.rst

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368321 91177308-0d34-0410-b5e6-96231b3b80d8