[MachineBasicBlock] Skip over debug instructions in computeRegisterLiveness...

[MachineBasicBlock] Skip over debug instructions in computeRegisterLiveness before checking for begin

If there are debug instructions before the stopping point,
we need to skip over them before checking for begin in order
to avoid having the debug instructions effect behavior.

Fixes PR43758.

Differential Revision: https://reviews.llvm.org/D69606