Revert r238473, "Thumb2: Modify codegen for memcpy intrinsic to prefer LDM/STM."

Revert r238473, "Thumb2: Modify codegen for memcpy intrinsic to prefer LDM/STM."
as it caused miscompilations and assertion failures (PR23768,
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150601/280380.html).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239169 91177308-0d34-0410-b5e6-96231b3b80d8