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