[llvm][SelectionDAGBuilder] codegen callbr.landingpad intrinsic

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.
Link: https://github.com/llvm/llvm-project/issues/59538

Part 3 from
https://discourse.llvm.org/t/rfc-syncing-asm-goto-with-outputs-with-gcc/65453/8

Follow up patches still need to wire up CallBrPrepare into the pass
pipelines.

Reviewed By: efriedma, void

Differential Revision: https://reviews.llvm.org/D140160