[VectorCombine] scalarize binop of inserted elements into vector constants

As with the extractelement patterns that are currently in vector-combine,
there are going to be several possible variations on this theme. This
should be the clearest, simplest example.

Scalarization is the right direction for target-independent canonicalization,
and InstCombine has some of those folds already, but it doesn't do this.
I proposed a similar transform in D50992. Here in vector-combine, we can
check the cost model to be sure it's profitable, so there should be less risk.

Differential Revision: https://reviews.llvm.org/D79452