[BOLT] Reduce the number of emitted symbols. NFCI. (#70175)

We emit a symbol before an instruction for a number of reasons, e.g. for
tracking LocSyms, debug line, or if the instruction has a label
annotation. Currently, we may emit multiple symbols per instruction.

Reuse the same label instead of creating and emitting new ones when
possible. I'm planning to refactor EH labels as well in a separate diff.

Change getLabel() to return a pointer instead of std::optional<> since
an empty label should be treated identically to no label.