[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
#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();
            });