[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