Merge r242733, r242734, r242735 and r242742

Merge r242733, r242734, r242735 and r242742

(r242742 is the interesting patch here, but I picked the others too to get a clean
 merge since there's been some back-and-forth on this file.)

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r242733 | matze | 2015-07-20 16:17:14 -0700 (Mon, 20 Jul 2015) | 3 lines

Revert "ARM: Use SpecificBumpPtrAllocator to fix leak introduced in r241920"

This reverts commit r241951. It caused http://llvm.org/PR24190
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r242734 | matze | 2015-07-20 16:17:16 -0700 (Mon, 20 Jul 2015) | 3 lines

Revert "ARMLoadStoreOpt: Merge subs/adds into LDRD/STRD; Factor out common code"

This reverts commit r241928. This caused http://llvm.org/PR24190
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r242735 | matze | 2015-07-20 16:17:20 -0700 (Mon, 20 Jul 2015) | 3 lines

Revert "ARMLoadStoreOptimizer: Create LDRD/STRD on thumb2"

This reverts commit r241926. This caused http://llvm.org/PR24190
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r242742 | matze | 2015-07-20 17:18:59 -0700 (Mon, 20 Jul 2015) | 7 lines

ARMLoadStoreOptimizer: Create LDRD/STRD on thumb2

Re-apply r241926 with an additional check that r13 and r15 are not used
for LDRD/STRD. See http://llvm.org/PR24190. This also already includes
the fix from r241951.

Differential Revision: http://reviews.llvm.org/D10623
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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_37@242907 91177308-0d34-0410-b5e6-96231b3b80d8