use ArgOperand accessors

and CallInst for getting hold
of the intrinsic's arguments

simplify along the way (at least for me this is much more legible now)
Bill, Baldrick or Anton, please review\!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106838 91177308-0d34-0410-b5e6-96231b3b80d8