[VectorCombine] Scalarize vector load/extract.

This patch adds a new combine that tries to scalarize chains of
`extractelement (load %ptr), %idx` to `load (gep %ptr, %idx)`. This is
profitable when extracting only a few elements out of a large vector.

At the moment, `store (extractelement (load %ptr), %idx), %ptr`
operations on large vectors result in huge code in the backend.

This can easily be triggered by using the matrix extension, e.g.
https://clang.godbolt.org/z/qsccPdPf4

This should complement D98240.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D100273