ARM: fix regression caused by r198914

The disassembler would no longer be able to disambiguage between the two
variants (explicit immediate #0 vs implicit, omitted #0) for the ldrt, strt,
ldrbt, strbt mnemonics as both versions indicated the disassembler routine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198944 91177308-0d34-0410-b5e6-96231b3b80d8