Partial rollback: Disable MLIR verifier parallelism.

[stellaraccident]

Summary

Deadlocks have been found in several downstream projects as noted on the original patch: https://reviews.llvm.org/D104207

Disabling pending full root cause analysis.

[rriddle]

The problems experienced in D104207 seem reasonable enough to do a temporary partial rollback until we fix it.

mlir/lib/IR/Verifier.cpp

 
 using namespace mlir;
 
 namespace {
 /// This class encapsulates all the state used to verify an operation region.
 class OperationVerifier {
 public:
   explicit OperationVerifier(MLIRContext *context)
-      : parallelismEnabled(context->isMultithreadingEnabled()) {}
+      // TODO: Re-enable parallelism once deadlocks found in D104207 are
+      // resolved.
+      : parallelismEnabled(false) {}
 
   /// Verify the given operation.
   LogicalResult verifyOpAndDominance(Operation &op);
 
 private:
   LogicalResult
   verifyBlock(Block &block,
               SmallVectorImpl<Operation *> &opsWithIsolatedRegions);