[InstCombine] replace shuffle's insertelement operand if inserted scalar is not demanded

This pattern is noted as a regression from:
D70246
...where we removed an over-aggressive shuffle simplification.

SimplifyDemandedVectorElts fails to catch this case when the insert has multiple uses,
so I'm proposing to pattern match the minimal sequence directly. This fold does not
conflict with any of our current shuffle undef/poison semantics.

Differential Revision: https://reviews.llvm.org/D71220