[InstCombine] shrink truncated splat shuffle

This is one part of solving a recent bug report:
http://lists.llvm.org/pipermail/llvm-dev/2017-February/110293.html

This keeps with our general approach: changing arbitrary shuffles is off-limts, 
but changing splat is ok. The transform is very similar to the existing 
shrinkBitwiseLogic() canonicalization.

Differential Revision: https://reviews.llvm.org/D30123

llvm-svn: 297155