[LV] Assert uniform recipes don't get predicated for when vectorizing.

Summary:
Add assertion ensuring invariant on construction, split off as suggested
from #98892.

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Differential Revision: https://phabricator.intern.facebook.com/D60250783

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8578,6 +8578,12 @@ VPReplicateRecipe *VPRecipeBuilder::handleReplication(Instruction *I,
     BlockInMask = getBlockInMask(I->getParent());
   }
 
+  // Note that there is some custom logic to mark some intrinsics as uniform
+  // manually above for scalable vectors, which this assert needs to account for
+  // as well.
+  assert((Range.Start.isScalar() || !IsUniform || !IsPredicated ||
+          (Range.Start.isScalable() && isa<IntrinsicInst>(I))) &&
+         "Should not predicate a uniform recipe");
   auto *Recipe = new VPReplicateRecipe(I, mapToVPValues(I->operands()),
                                        IsUniform, BlockInMask);
   return Recipe;