[AsmPrinter] fix nullptr dereference for MBBs with hasAddressTaken property without BB

Basic block pointer is dereferenced unconditionally for MBBs with
hasAddressTaken property.

MBBs might have hasAddressTaken property without reference to BB.
Backend developers must assign fake BB to MBB to workaround this issue
and it should be fixed.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D108092