[IR] Convert check to assertion in PredIterator (NFC) (llvm#137931)

After llvm#137799 and
llvm#137816, instruction uses of
BasicBlocks are always terminators, so assert that instead of checking
it.

Author: nikic

llvm/include/llvm/IR/CFG.h

@@ -54,9 +54,10 @@ class PredIterator {
   inline void advancePastNonTerminators() {
     // Loop to ignore non-terminator uses (for example BlockAddresses).
     while (!It.atEnd()) {
-      if (auto *Inst = dyn_cast<Instruction>(*It))
-        if (Inst->isTerminator())
-          break;
+      if (auto *Inst = dyn_cast<Instruction>(*It)) {
+        assert(Inst->isTerminator() && "BasicBlock used in non-terminator");
+        break;
+      }
 
       ++It;
     }