[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 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "[Thumb/Thumb2] Added restrictions on PC, LR, SP in the register list for PUSH/POP/LDM/STM. <Differential Revision: http://reviews.llvm.org/D6090>"

This reverts commit r223356.

It was failing check-all (MC/ARM/thumb.s in particular).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223363 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r223347 which has caused crashes on bootstrap bots.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223364 91177308-0d34-0410-b5e6-96231b3b80d8