[DAGCombiner] fold extract_subvector of extract_subvector

This is the sibling fold for insert-of-insert that was added with D56604.

Now that we have x86 shuffle narrowing (D57156), this change shows improvements for 
lots of AVX512 reduction code (not sure that we would ever expect extract-of-extract otherwise).

There's a small regression in some of the partial-permute tests (extracting followed by splat).
That is tracked by PR40500:
https://bugs.llvm.org/show_bug.cgi?id=40500

Differential Revision: https://reviews.llvm.org/D57336

llvm-svn: 352528

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