Make ARMAsmParser accept the correct alignment specifier syntax in instructions.

The parser will now accept instructions with alignment specifiers written like
    vld1.8  {d16}, [r0:64]
, while also still accepting the incorrect syntax
    vld1.8  {d16}, [r0, :64]



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175164 91177308-0d34-0410-b5e6-96231b3b80d8