Fix a minor regression introduced in r223113

r223113 added support for ARM modified immediate assembly syntax. That patch
has broken support for immediate expressions, as in:
    add r0, #(4 * 4)
It wasn't caught because we don't have any tests for this feature. This patch
fixes this regression and adds test cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223366 91177308-0d34-0410-b5e6-96231b3b80d8