SIL: remove dangerous SILBasicBlock APIs

`isSuccessorBlock` and `isPredecessorBlock` are dangerous because they can easily lead to quadratic behavior.
Fortunately they are not used anywhere (except in one place for verification, which I rewrote).

Author: eeckstein

include/swift/SIL/SILBasicBlock.h

@@ -427,11 +427,6 @@ public SwiftObjectHeader {
     return getTerminator()->getSingleSuccessorBlock();
   }
 
-  /// Returns true if \p BB is a successor of this block.
-  bool isSuccessorBlock(SILBasicBlock *Block) const {
-    return getTerminator()->isSuccessorBlock(Block);
-  }
-
   using SuccessorBlockListTy = TermInst::SuccessorBlockListTy;
   using ConstSuccessorBlockListTy = TermInst::ConstSuccessorBlockListTy;
 
@@ -459,12 +454,6 @@ public SwiftObjectHeader {
     return {pred_begin(), pred_end()};
   }
 
-  bool isPredecessorBlock(SILBasicBlock *BB) const {
-    return any_of(
-        getPredecessorBlocks(),
-        [&BB](const SILBasicBlock *PredBB) -> bool { return BB == PredBB; });
-  }
-
   SILBasicBlock *getSinglePredecessorBlock() {
     if (pred_empty() || std::next(pred_begin()) != pred_end())
       return nullptr;

include/swift/SIL/SILInstruction.h

@@ -8940,14 +8940,6 @@ class TermInst : public NonValueInstruction {
     return const_cast<TermInst *>(this)->getSingleSuccessorBlock();
   }
 
-  /// Returns true if \p BB is a successor of this block.
-  bool isSuccessorBlock(SILBasicBlock *BB) const {
-    auto Range = getSuccessorBlocks();
-    return any_of(Range, [&BB](const SILBasicBlock *SuccBB) -> bool {
-      return BB == SuccBB;
-    });
-  }
-
   using SuccessorBlockArgumentListTy =
       TransformRange<ConstSuccessorListTy, function_ref<ArrayRef<SILArgument *>(
                                                const SILSuccessor &)>>;

lib/SILOptimizer/Analysis/LoopRegionAnalysis.cpp

@@ -218,10 +218,12 @@ void LoopRegionFunctionInfo::verify() {
 
       // If R and OtherR are blocks, then OtherR should be a successor of the
       // real block.
-      if (R->isBlock() && OtherR->isBlock())
-        assert(R->getBlock()->isSuccessorBlock(OtherR->getBlock()) &&
+      if (R->isBlock() && OtherR->isBlock()) {
+        auto succs = R->getBlock()->getSuccessors();
+        assert(std::find(succs.begin(), succs.end(), OtherR->getBlock()) != succs.end() &&
                "Expected either R was not a block or OtherR was a CFG level "
                "successor of R.");
+      }
     }
   }
 #endif