Add parsing for the Thumb t_addrmode_s4 addressing mode. This can almost

certainly be made more generic. But it does allow us to parse something like:

          ldr     r3, [r2, r4]

correctly in Thumb mode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120408 91177308-0d34-0410-b5e6-96231b3b80d8