$ cat t.c
a, b, c;
fn1() {
  while (b--) {
    c = a;
    switch (a & 3)
    case 0:
    do
    case 3:
  case 2:
  case 1:
    ;
    while (--c)
      ;
  }
}

$ clang -O2 t.c
...
#8 0x00000000019ffbaa (anonymous namespace)::InnerLoopVectorizer::createBlockInMask(llvm::BasicBlock*) (bin/clang)
#9 0x0000000001a000d4 (anonymous namespace)::InnerLoopVectorizer::createEdgeMask(llvm::BasicBlock*, llvm::BasicBlock*) (bin/clang)
... repeats 100+ times ...
1.      <eof> parser at end of file
2.      Per-module optimization passes
3.      Running pass 'Function Pass Manager' on module 't.c'.
4.      Running pass 'Loop Vectorization' on function '@fn1'
clang: error: unable to execute command: Segmentation fault (core dumped)
clang: error: clang frontend command failed due to signal (use -v to see invocation)
clang version 3.9.0 (trunk 275303) (llvm/trunk 275304)

The crash only occurs at -O2, the bug comes from the interaction of multiple passes.