Function create_temp_if_necessary 

fn create_temp_if_necessary<'tcx, I: Inliner<'tcx>>(
    inliner: &I,
    arg: Operand<'tcx>,
    callsite: &CallSite<'tcx>,
    caller_body: &mut Body<'tcx>,
    return_block: Option<BasicBlock>,
) -> Local

If arg is already a temporary, returns it. Otherwise, introduces a fresh temporary T and an instruction T = arg, and returns T.