D78586 implements good sanity checks in MachineVerifier that for each
LiveIn register to a MachineBasicBlock, that at least one predecessor
MachineBasicBlock defines the register as LiveOut.
The PhysReg MachineOperands for INLINEASM_BR MachineInstrs were being
implicit-def'd but marked dead (i.e. no uses in local MachineBasicBlock)
ScheduleDAGSDNodes::EmitSchedule() splits the MachineBasicBlock
containing the INLINEASM_BR, post that MachineInstr, and marks the
LiveIns. But there's no marking of LiveOuts. If a use is visible within
the same MachineBasicBlock, live-vars will calculate the LiveOuts
Since the uses aren't visible in the same MachineBasicBlock as the
INLINEASM_BR MachineInstr, teach LiveVariables::runOnBlock() to special
case INLINEASM_BR's PhysReg defs, so that they're not marked dead.
Implementing TCOPY (D75098) should also make the uses visible to
live-vars as well, at which point, this change to LiveVariables should
be reverted (but not the tests).