Fix broken build: change switch cast to use llvm_unreachable.

PiperOrigin-RevId: 206170570
diff --git a/lib/IR/Instructions.cpp b/lib/IR/Instructions.cpp
index 7afaf68..fa578f1 100644
--- a/lib/IR/Instructions.cpp
+++ b/lib/IR/Instructions.cpp
@@ -227,7 +227,7 @@
 MutableArrayRef<BBDestination> TerminatorInst::getDestinations() {
   switch (getKind()) {
   case Kind::Operation:
-    assert(0 && "not a terminator");
+    llvm_unreachable("not a terminator");
   case Kind::Branch:
     return cast<BranchInst>(this)->getDestinations();
   case Kind::CondBranch: