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
ARM IAS: support GNU extension for ldrd, strd

The GNU assembler has an extension that allows for the elision of the paired
register (dt2) for the LDRD and STRD mnemonics.  Add support for this in the
assembly parser.  Canonicalise the usage during the instruction parsing from
the specified version.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198915 91177308-0d34-0410-b5e6-96231b3b80d8
ARM IAS: support #:{lower,upper}16: for GNU compatibility

The GNU assembler supports prefixing the expression with a '#' to indiciate that
the value that is being moved is infact a constant.  This improves the
compatibility of the integrated assembler's parser for this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198916 91177308-0d34-0410-b5e6-96231b3b80d8