[Thumb2] LDRS?[BH] cannot load to the PC

The Thumb2 LDRS?[BH] instructions are not valid when the destination
register is the PC (these encodings are used for preload hints).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220278 91177308-0d34-0410-b5e6-96231b3b80d8