[llvm] [Parallel] Revert sequential task changes (PR #109084)

[Fangrui Song via llvm-commits]

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@@ -217,13 +194,18 @@ TaskGroup::~TaskGroup() {
 void TaskGroup::spawn(std::function<void()> F, bool Sequential) {
 #if LLVM_ENABLE_THREADS
   if (Parallel) {
+    if (Sequential) {
+      // Act as worker thread 0.
+      threadIndex = 0;
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MaskRay wrote:

Thanks. I just realized the flipped condition :) Adopted the style and kept the `threadIndex != UINT_MAX` assertion.

https://github.com/llvm/llvm-project/pull/109084