[ConstraintElim] Move after first instcombine run.

[fhahn]

Summary

Running ConstraintEliminiation after the first InstCombine run results
in slightly more simplifications on average.

There are is a tiny number of regressions, mostly due to CVP eliminating
a condition that ConstraintElimination would use, but in most cases
there's a slight improvement or no change.

[nikic]

LG

There are is a tiny number of regressions, mostly due to CVP eliminating
a condition that ConstraintElimination would use, but in most cases
there's a slight improvement or no change.

To clarify, the condition could still be derived from earlier ones, but ConstraintElimination just isn't powerful enough to do that? (E.g. because it requires reasoning about ranges.)

llvm/lib/Passes/PassBuilderPipelines.cpp

PassBuilder::buildFunctionSimplificationPipeline(OptimizationLevel Level,
 
   // Global value numbering based sinking.
   if (EnableGVNSink) {
     FPM.addPass(GVNSinkPass());
     FPM.addPass(
         SimplifyCFGPass(SimplifyCFGOptions().convertSwitchRangeToICmp(true)));
   }
 
-  if (EnableConstraintElimination)
-    FPM.addPass(ConstraintEliminationPass());
-
   // Speculative execution if the target has divergent branches; otherwise nop.
   FPM.addPass(SpeculativeExecutionPass(/* OnlyIfDivergentTarget =*/true));
 
   // Optimize based on known information about branches, and cleanup afterward.
   FPM.addPass(JumpThreadingPass());
   FPM.addPass(CorrelatedValuePropagationPass());
 
   FPM.addPass(
       SimplifyCFGPass(SimplifyCFGOptions().convertSwitchRangeToICmp(true)));
   FPM.addPass(InstCombinePass());
   if (Level == OptimizationLevel::O3)
     FPM.addPass(AggressiveInstCombinePass());
 
+  if (EnableConstraintElimination)
+    FPM.addPass(ConstraintEliminationPass());
+
   if (!Level.isOptimizingForSize())
     FPM.addPass(LibCallsShrinkWrapPass());
 
   invokePeepholeEPCallbacks(FPM, Level);
 
   // For PGO use pipeline, try to optimize memory intrinsics such as memcpy
   // using the size value profile. Don't perform this when optimizing for size.
   if (PGOOpt && PGOOpt->Action == PGOOptions::IRUse &&