Add set reverse block utility

enzyme/Enzyme/CApi.cpp

diff --git a/enzyme/Enzyme/CApi.cpp b/enzyme/Enzyme/CApi.cpp
index df4a6b2..5ac78c2 100644
--- a/enzyme/Enzyme/CApi.cpp
+++ b/enzyme/Enzyme/CApi.cpp
@@ -606,6 +606,16 @@
                                       forkCache, push));
 }
 
+void EnzymeGradientUtilsSetReverseBlock(GradientUtils *gutils,
+                                                     LLVMBasicBlockRef block) {
+  auto endBlock = cast<BasicBlock>(unwrap(block));
+  auto found = gutils->reverseBlockToPrimal.find(endBlock);
+  assert(found != gutils->reverseBlockToPrimal.end());
+  auto &vec = gutils->reverseBlocks[found->second];
+  assert(vec.size());
+  vec.push_back(endBlock);
+}
+
 LLVMValueRef EnzymeCreateForwardDiff(
     EnzymeLogicRef Logic, LLVMValueRef request_req, LLVMBuilderRef request_ip,
     LLVMValueRef todiff, CDIFFE_TYPE retType, CDIFFE_TYPE *constant_args,