[InstCombine] shrink truncated insertelement into undef vector

This is the 2nd part of solving:
http://lists.llvm.org/pipermail/llvm-dev/2017-February/110293.html

D30123 moves the trunc ahead of the shuffle, and this moves the trunc ahead of the insertelement. 
We're limiting this transform to undef rather than any constant to avoid backend problems.

Differential Revision: https://reviews.llvm.org/D30137

llvm-svn: 297242