[SelectionDAG] fold concat of extract subvectors

This is derived from the related fold for build vectors.
We also have a version of this in DAGCombiner. The benefit of
having this fold at node creation time is (1) efficiency and
(2) preventing infinite looping from creating patterns that
should not exist in the first place.

Currently, the inf-loop could happen with MergeConsecutiveStores()
because it naively creates concat of extracts when forming a wider
vector store. That could fight with target-specific store narrowing.

llvm-svn: 361780

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