[llvm] Fix region simplification bug (NVIDIA#1889)

docker/build/devdeps.Dockerfile

@@ -52,6 +52,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends git \
 ADD ./tpls/customizations/llvm/llvm_pr71968_mod.diff /
 RUN cd /llvm-project && git apply /llvm_pr71968_mod.diff && rm /llvm_pr71968_mod.diff
 
+# Apply customization for https://github.com/NVIDIA/cuda-quantum/issues/1799
+ADD ./tpls/customizations/llvm/fix_region_simplification.diff /
+RUN cd /llvm-project && git apply /fix_region_simplification.diff && rm /fix_region_simplification.diff
+
 # Build the the LLVM libraries and compiler toolchain needed to build CUDA-Q;
 # The safest option to avoid any compatibility issues is to build an application using these libraries 
 # with the same compiler toolchain that the libraries were compiled with.

tpls/customizations/llvm/fix_region_simplification.diff

@@ -0,0 +1,20 @@
+diff --git a/mlir/lib/Transforms/Utils/RegionUtils.cpp b/mlir/lib/Transforms/Utils/RegionUtils.cpp
+index 996588243..63ab385be 100644
+--- a/mlir/lib/Transforms/Utils/RegionUtils.cpp
++++ b/mlir/lib/Transforms/Utils/RegionUtils.cpp
+@@ -679,6 +679,15 @@ static LogicalResult mergeIdenticalBlocks(RewriterBase &rewriter,
+       if (hasNonEmptyRegion)
+         continue;
+
++      // Don't allow merging if this block's arguments are used outside of the
++      // original block.
++      bool argHasExternalUsers = llvm::any_of(
++          block->getArguments(), [block](mlir::BlockArgument &arg) {
++            return arg.isUsedOutsideOfBlock(block);
++          });
++      if (argHasExternalUsers)
++        continue;
++
+       // Try to add this block to an existing cluster.
+       bool addedToCluster = false;
+       for (auto &cluster : clusters)