[InstCombine] avoid infinite loop from partial undef vectors

This fixes the examples from
D99674 and
https://llvm.org/PR49878

The matchers succeed on partial undef/poison vector constants,
but the transform creates a full 'not' (-1) constant, so it
would undo a demanded vector elements change triggered by the
extractelement.

Differential Revision: https://reviews.llvm.org/D100044