[sdag] Add the helper I most want to the DAG -- building a bitcast

[sdag] Add the helper I most want to the DAG -- building a bitcast
around a value using its existing SDLoc.

Start using this in just one function to save omg lines of code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238638 91177308-0d34-0410-b5e6-96231b3b80d8