[InstCombine] enhance vector demanded elements to look at a vector select condition operand

I noticed that we were not back-propagating undef lanes to shuffle masks when we have a 
shuffle that reduces the vector width. This is part of investigating/solving PR38691:
https://bugs.llvm.org/show_bug.cgi?id=38691

The DAG equivalent was proposed with:
D51696

Differential Revision: https://reviews.llvm.org/D51433

llvm-svn: 341981