This simplifies code in SelectionDAGBuilder. Originally, this intrinsic
was designed to have an implicit operand, based on
https://gist.github.com/nickdesaulniers/6df8e1a15f23636e2bf8abec96d425c7.
Turns out, codegen is simpler to just have an explicit parameter. Make
what was the implicit corresponding CallBrInst explicit and simplify
codegen.