[VPlan] Add narrowToSingleScalarRecipe transform. (#139150)

Add a new convertToUniformRecipes transform which uses VPlan-based
uniformity analysis to determine if wide recipes and replicate recipes
can be converted to uniform recipes.

There are a few places where we ad-hoc convert recipes to uniform
recipes, which this transform will eventually replace. There are a few
more generalizations required to do so which I plan to do as follow-ups.

By converting the recipes to uniform recipes, we effectively materialize
the information from the VPlan-based analysis.

Note that there is one regression at the moment in SystemZ/pr47665.ll
due to trivial constant folding opportunities in the input IR. This will
be fixed by VPlan-based constant folding
(https://github.com/llvm/llvm-project/pull/125365/)

PR: https://github.com/llvm/llvm-project/pull/139150

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