Steps to reproduce:

$ llvm/Debug/bin/opt -etforest -break-crit-edges -domtree x.bc -disable-output
Segmentation fault

The backtrace shows an infinite loop
#123147 0x085b20b7 in llvm::DominatorTree::getNodeForBlock (this=0x87191f8, 
    BB=0x0) at Dominators.cpp:380

but before that, it  seems that llvm::DominatorTree::calculate's "ImmDom", which
is gets from ImmediateDominator is out of date:

#174577 0x085b225f in llvm::DominatorTree::calculate (this=0x87191f8, 
    ID=@0x871a1a8) at Dominators.cpp:399
399             Node *IDomNode = getNodeForBlock(ImmDom);
(gdb) p ImmDom
$2 = (class llvm::BasicBlock *) 0x8719838
(gdb) p ImmDom->getName()
Cannot access memory at address 0xff75effc
(gdb) down
#174576 0x085b20b7 in llvm::DominatorTree::getNodeForBlock (this=0x87191f8, 
    BB=0x8719838) at Dominators.cpp:380
380       Node *IDomNode = getNodeForBlock(IDom);
(gdb) p IDom
$3 = (class llvm::BasicBlock *) 0x0

and that's the start of the infinite loop.