[VectorCombine] allow peeking through an extractelt when creating a vector load

This is an enhancement to load vectorization that is motivated by
a pattern in https://llvm.org/PR16739.
Unfortunately, it's still not enough to make a difference there.
We will have to handle multi-use cases in some better way to avoid
creating multiple overlapping loads.

Differential Revision: https://reviews.llvm.org/D92858