[BOLT] Fix segfault in debug print

Summary:
With "-debug" flag we are using a dump in intermediate state when
basic block's list is initialized, but layout is not. In new isSplit()
funciton we were checking the size() which uses basic block list,
and then we were accessing the (uninitiazed) layout.
Instead of checking size() we should be checking layout_size().

(cherry picked from FBD6277770)