As part of re-architecting callbr to no longer use blockaddresses
(https://reviews.llvm.org/D129288), we don't really need them in MIR.
They make comparing MachineBasicBlocks of indirect targets during
MachineVerifier a PITA.
Suggested by @efriedma from the discussion:
https://reviews.llvm.org/D130290#3669531
This is a roundabout way of saying cast<CallBrInst>(&Call)->getIndirectDest(LabelNo) -- unless the fact that a BlockAddress on the IR level is created is still important for something?