[BOLT] Fix branch data for __builtin_unreachable().

Summary:
When we have a conditional branch past the end of function (a result
of a call to__builtin_unreachable()), we replace the branch with nop,
but keep branch information for validation purposes. If that branch
has a recorded profile we mistakenly create an additional successor
to a containing basic block (a 3rd successor).

Instead of adding the branch to FTBranches list we should be adding
to IgnoredBranches.

(cherry picked from FBD4912840)