[X86] Improve a dag-combine that handles a vector extract -> zext sequence.
The current DAG combine turns a sequence of extracts from <4 x i32> followed by zexts into a store followed by scalar loads.
According to measurements by Martin Krastev (see PR 21269) for x86-64, a sequence of an extract, movs and shifts gives better performance. However, for 32-bit x86, the previous sequence still seems better.
Differential Revision: http://reviews.llvm.org/D6501
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223360
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