InstCombine: form shuffles from wider range of insert/extractelements

Sequences of insertelement/extractelements are sometimes used to build
vectorsr; this code tries to put them back together into shuffles, but
could only produce a completely uniform shuffle types (<N x T> from two
<N x T> sources).

This should allow shuffles with different numbers of elements on the
input and output sides as well.

llvm-svn: 203229