This patch introduces a new ComputeReductionResult opcode to compute the
final reduction result in the middle block. The code from fixReduction
has been moved to ComputeReductionResult, after some earlier cleanup
changes to model parts of fixReduction explicitly elsewhere as needed.
The recipe may be broken down further in the future.
Note that the phi nodes to merge the reduction result from the trip
count check and the middle block, to be used as resume value for the
scalar remainder loop are also generated based on
ComputeReductionResult.
Once we have a VPValue for the reduction result, this can also be
modeled explicitly and moved out of the recipe.
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