ARM IAS: support implicit immediate 0s for {LD,ST}R{B,}T

ARM IAS: support implicit immediate 0s for {LD,ST}R{B,}T

The ARM ARM indicates the mnemonics as follows:

  ldrbt{<c>}{<q>} <Rt>, [<Rn>], {, #+/-<imm>}
  ldrt{<c>}{<q>} <Rt>, [<Rn>] {, #+/-<imm>}
  strbt{<c>}{<q>} <Rt>, [<Rn>] {, #<imm>}
  strt{<c>}{<q>} <Rt>, [<Rn>] {, #+/-<imm>}

This improves the parser to deal with the implicit immediate 0 for the mnemonics
as per the specification.

Thanks to Joerg Sonnenberger for the tests!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198914 91177308-0d34-0410-b5e6-96231b3b80d8