[DebugInfo] Re-apply two patches to MachineSink

These were:
 * D58386 / f5e1b718 / reverted in d382a8a7
 * D58238 / ee50590e / reverted in a8db456b

Of which the latter has a performance regression tracked in PR43855,
fixed by D70672 / D70676, which will be committed atomically with this
reapplication.

Contains a minor difference to account for a change in the IsCopyInstr
signature.