[FastISel] Breakout intrinsic lowering into a separate function and add a target-hook.

Create a separate helper function for target-independent intrinsic lowering. Also
add an target-hook that allows to directly call into a target-sepcific intrinsic
lowering method. Currently the implementation is opt-in and doesn't affect
existing target implementations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212843 91177308-0d34-0410-b5e6-96231b3b80d8