[llvm] [GlobalISel] Don't treat intrinsic callbr indirect dest as machine successor. (PR #221685)

[Matt Arsenault via llvm-commits]

================
@@ -240,18 +240,27 @@ bool InstructionSelectImpl::selectMachineFunction(MachineFunction &MF) {
   }
 
   for (MachineBasicBlock &MBB : MF) {
-    if (MBB.empty())
-      continue;
-
     if (!SelectedBlocks.contains(&MBB)) {
       // This is an unreachable block and therefore hasn't been selected, since
       // the main selection loop above uses a postorder block traversal.
       // We delete all the instructions in this block since it's unreachable.
       MBB.clear();
+
+      // Clearing dropped the terminators that encoded this block's successors,
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arsenm wrote:

Such dropping shouldn't be possible in the selector 

https://github.com/llvm/llvm-project/pull/221685