CodeGen: refactor ARM builtin handling

Create a helper function to construct a value for the ARM hint intrinsic
rather than inling the construction.  In order to avoid the use of the sentinel
value, inline the use of intrinsic instruction retrieval.  NFC.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@223338 91177308-0d34-0410-b5e6-96231b3b80d8