ARM IAS: properly handle expression operands

Operands which involved label arithemetic would previously fail to parse.  This
corrects that by adding the additional case for the shift operand validation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198735 91177308-0d34-0410-b5e6-96231b3b80d8