[BOLT] Explicitly check for returns when extending call continuation profile

[aaupov]

Call continuation logic relies on assumptions about fall-through origin:

• the branch is external to the function,
• fall-through start is at the beginning of the block,
• the block is not an entry point or a landing pad.

Leverage trace information to explicitly check whether the origin is a
return instruction, and defer to checks above only in case of
DSO-external branch source.

This covers both regular and BAT cases, addressing call continuation
fall-through undercounting in the latter mode.

Test Plan: TBD