[MachineBB] Make sure there are successors in terminatorIsComputedGoto.

Currently terminatorIsComputedGoto will return for blocks with a
indirect branch terminator and no successor. If there are no successor,
the terminator is likely not a computed goto, return false in that case.

Note that this is currently NFC, as the only use checks it only if there
are successors, but it will be needed in
llvm#150911.

Author: fhahn

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -326,7 +326,7 @@ class MachineBasicBlock
   /// Returns true if the original IR terminator is an `indirectbr`. This
   /// typically corresponds to a `goto` in C, rather than jump tables.
   bool terminatorIsComputedGoto() const {
-    return back().isIndirectBranch() &&
+    return back().isIndirectBranch() && !succ_empty() &&
            llvm::all_of(successors(), [](const MachineBasicBlock *Succ) {
              return Succ->isIRBlockAddressTaken();
            });