Perform mergefunc after constmerge

Constmerge can fold switch jump tables, possibly making functions
identical again. It can help mergefunc.
On the otherhand, the opposite seems unlikely.

Fixes #92201

Author: yamt

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -1525,17 +1525,18 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   if (EnableIROutliner)
     MPM.addPass(IROutlinerPass());
 
-  // Merge functions if requested.
-  if (PTO.MergeFunctions)
-    MPM.addPass(MergeFunctionsPass());
-
   // Now we need to do some global optimization transforms.
   // FIXME: It would seem like these should come first in the optimization
   // pipeline and maybe be the bottom of the canonicalization pipeline? Weird
   // ordering here.
   MPM.addPass(GlobalDCEPass());
   MPM.addPass(ConstantMergePass());
 
+  // Merge functions if requested. It has a better chance to merge functions
+  // after ConstantMerge folded jump tables.
+  if (PTO.MergeFunctions)
+    MPM.addPass(MergeFunctionsPass());
+
   if (PTO.CallGraphProfile && !LTOPreLink)
     MPM.addPass(CGProfilePass(LTOPhase == ThinOrFullLTOPhase::FullLTOPostLink ||
                               LTOPhase == ThinOrFullLTOPhase::ThinLTOPostLink));