[BOLT] Allow function entry to be a cold fragment

Allow cold fragment to get new address.

Our previous assumption is that a fragment (.cold) is only reached
through the main fragment of same function. In addition, .cold fragment
must be reached through either (a) direct transfer, or (b) split jump
table. For (a), we perform a simple fix-up. For (b), we currently mark
all relevant fragments as non-simple. Therefore, there is no need to
get new address for .cold fragment.

This is not always the case, as function entry can be rarely executed,
and is placed in .text.cold segment. Essentially we cannot tell which
the source-level function entry is based on hot and cold segments,
so we must treat each fragment a function on its own. Therfore, we
remove the assertion that a function entry cannot be cold fragment.

Test Plan:
```
ninja check-bolt
```

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D128111