[FastISel] Make isInTailCallPosition independent of SelectionDAG.

Break out the arguemnts required from SelectionDAG, so that this function can
also be used by FastISel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212844 91177308-0d34-0410-b5e6-96231b3b80d8