change llvm::MergeBlockIntoPredecessor to not merge two blocks BB1->BB2

when BB2 has its address taken.  Since it ends up doing BB2->rauw(BB1),
this can cause the address of the entry block to be taken.  Since it is
generally undesirable to nuke blocks whose address is taken, even when
we can, just unconditionally stop this xform.

llvm-svn: 85708