[GVN] lazily update dominator tree when merge unconditional branches …

…in function (#88477)

With this change, the wall time for of GVN pass decreased from
873,745.492 ms to 367,375.304 ms in an our internal testcase.

llvm/lib/Transforms/Scalar/GVN.cpp

@@ -2708,7 +2708,7 @@ bool GVNPass::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
   bool Changed = false;
   bool ShouldContinue = true;
 
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
   // Merge unconditional branches, allowing PRE to catch more
   // optimization opportunities.
   for (BasicBlock &BB : llvm::make_early_inc_range(F)) {
@@ -2718,6 +2718,7 @@ bool GVNPass::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
 
     Changed |= removedBlock;
   }
+  DTU.flush();
 
   unsigned Iteration = 0;
   while (ShouldContinue) {