[VectorCombine] Try to reduce shuffle cost for commutative reduction operands

Given a shuffle feeding a commutative reduction, the lane ordering of
the shuffle will not alter the result. This is also true if there are a
number of operations between the reduction and the shuffle, providing
they only operate lane-wise. This patch searches for cases like that in
Vector Combine, allowing us to check the cost of the shuffle vs an
in-order identity shuffle and replace the order if possible. This only
handles a single shuffle at the moment to keep things simple, and is
able to ignore splats that produce results where every result is the
same.

This is a more powerful version of a combine that already happens in
instrcombine, capable of optimizing more cases by looking through more
instructions and being able to cost the shuffle.

Differential Revision: https://reviews.llvm.org/D123494

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