Document GlobalISel's combiner guidelines

[aemerson]

@tschuett made a good point that our approach to writing combiners in GlobalISel needs some documentation here: #91922

This issue is to track that initial work on elaborating things like:

• Combiner best practices
• Rationales for what we do
• Artifact vs regular combiner
• Notable exceptions to the above and why they're there

[tschuett]

I vote for a section about the combiner design. We compile the Combine.td into a matchable. 10 more lines.

Complex logic is delegated to CombinerHelper Helper.

From this design, we can conclude:

• Keep as much logic as possible in Combine.td and less logic in the Helper => lower compile time.
• visit functions are an anti pattern. They delegate too much logic into the helper => higher compile time.
• Many tight, precise, and small combines for lower compile time and targets can pick the choices.