[InstCombine] insert a new shuffle before its uses (PR26015)

Although this solves the test case in PR26015:
https://llvm.org/bugs/show_bug.cgi?id=26015

And may solve PR25999:
https://llvm.org/bugs/show_bug.cgi?id=25999

...I suspect this is not the best solution. I think we want to insert the new shuffle
just ahead of the earliest ExtractElementInst that we're replacing, but I don't know 
how that should be implemented.

Differential Revision: http://reviews.llvm.org/D15878

llvm-svn: 256857