[MachineBB] Make sure there are successors in terminatorIsComputedGoto. (#151342)

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.

PR: #151342

Author: fhahn

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -323,10 +323,11 @@ class MachineBasicBlock
   const MachineFunction *getParent() const { return xParent; }
   MachineFunction *getParent() { return xParent; }
 
-  /// 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() &&
+  /// Returns true if the original IR terminator is an `indirectbr` with
+  /// successor blocks. This typically corresponds to a `goto` in C, rather than
+  /// jump tables.
+  bool terminatorIsComputedGotoWithSuccessors() const {
+    return back().isIndirectBranch() && !succ_empty() &&
            llvm::all_of(successors(), [](const MachineBasicBlock *Succ) {
              return Succ->isIRBlockAddressTaken();
            });

llvm/lib/CodeGen/TailDuplicator.cpp

@@ -604,7 +604,7 @@ bool TailDuplicator::shouldTailDuplicate(bool IsSimple,
   bool HasComputedGoto = false;
   if (!TailBB.empty()) {
     HasIndirectbr = TailBB.back().isIndirectBranch();
-    HasComputedGoto = TailBB.terminatorIsComputedGoto();
+    HasComputedGoto = TailBB.terminatorIsComputedGotoWithSuccessors();
   }
 
   if (HasIndirectbr && PreRegAlloc)