Fix 64-bit atomic operations in Thumb mode.

The ARM and Thumb variants of LDREXD and STREXD have different constraints and
take different operands. Previously the code expanding atomic operations didn't
take this into account and asserted in Thumb mode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173780 91177308-0d34-0410-b5e6-96231b3b80d8