[llvm] [VPlan] Add convertToUniformRecipe transform. (PR #139150)

[via llvm-commits]

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@@ -1086,6 +1086,40 @@ void VPlanTransforms::simplifyRecipes(VPlan &Plan, Type &CanonicalIVTy) {
   }
 }
 
+static void convertToUniformRecipes(VPlan &Plan) {
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ayalz wrote:

```suggestion
static void convertToSingleScalarRecipes(VPlan &Plan) {
```
as this captures both uniformity and only-first-lane-used? Also affects title of patch.

Analogous to `truncateToMinimalBitwidths()` which aims to reduce each lane to fewer bits, this aims to reduce each part to fewest lanes - to one. Perhaps both should start with `narrow`, as used in the now inlined lambda.

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