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
Hopefully fix the Windows build failure introduced in r173769

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173781 91177308-0d34-0410-b5e6-96231b3b80d8