CodeGen: Represent block arguments on MachineBasicBlock
The receiver half of the block-argument representation: a block declares a list
of virtual registers that receive values forwarded by each predecessor's
SUCC_ARGS. Unlike a PHI, a block argument is defined by the block itself, not by
an instruction.
MachineBasicBlock holds the argument list, printed and parsed as an "arguments:"
block header line. MachineRegisterInfo maps each argument to its defining block,
so getDefBlock works for these registers. The verifier treats an argument as
defined at block entry and requires each predecessor to supply one matching
SUCC_ARGS.
This is only building infrastructure, and is not yet used.
Co-Authored-By: Claude <noreply@anthropic.com> (Claude Opus 4.8)
