[InstCombine] allow undef elements when forming splat from chain of insertelements
We allow forming a splat (broadcast) shuffle, but we were conservatively limiting
that to cases where all elements of the vector are specified. It should be safe
from a codegen perspective to allow undefined lanes of the vector because the
expansion of a splat shuffle would become the chain of inserts again.
Differential Revision: https://reviews.llvm.org/D63848