[llvm] [VPlan] Add additional guiding principles to docs. (PR #85688)

[Nikolay Panchenko via llvm-commits]

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@@ -88,6 +88,33 @@ The design of VPlan follows several high-level guidelines:
    detection and formation involves searching for and optimizing instruction
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npanchen wrote:

nit: cannot comment on a line 86, but I assume `Step 2.b` should really be ` Step 2.ii`

https://github.com/llvm/llvm-project/pull/85688