This patch updates DuplicateInstructionsInSplitBetween to update a DTU
instead of applying updates to the DT directly.
Given that there only are 2 users, I also updated them in this patch to
I slightly moved the code in CallSiteSplitting around to reduce the
places where we have to pass in DTU. If necessary, I could split those
changes in a separate patch.
This fixes missing DT updates when dealing with musttail calls in
CallSiteSplitting, by using DTU->deleteBB.