patch: make instcombine remove shuffles by reordering vector elements

[Nick Lewycky]

This patch teaches instcombine to detect cases where we can change the 
construction of a vector to change the order of the elements, and 
consequently remove the need for a shufflevector operation later on. 
Please review!

It would be nice in particular if the folks working on vectorization 
would try it out. One thing I haven't even tried is matching horizontal 
operations as a place to start the "can evaluate in different element 
order?" machinery.

Nick
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