Given a CallBrInst, retain its first virtual register in SelectionDagBuilder's
FunctionLoweringInfo if there's corresponding landingpad. Walk the list
of COPY MachineInstr to find the original virtual and physical registers
defined by the INLINEASM_BR MachineInst.
Test cases from https://reviews.llvm.org/D139565.
Part 3 from
Follow up patches still need to wire up CallBrPrepare into the pass