Follow on from D112725, which changes VPReplicateRecipe to generate the
last lane of a uniform store. This patch changes collectLoopUniforms to add stores
to the list of uniform instructions where Legal->isUniformMemOp is true.
setCostBasedWideningDecision now sets the widening decision for all uniform
memory ops to Scalarize, where previously GatherScatter may have been
chosen for scalable stores.