[IRCE] Avoid computing potentially unnecessary analyses. NFC

[anna]

Summary

IRCE is a function pass that operates on loops. If there are no loops in
the function (as seen through LI), we should avoid computing the
remaining expensive analyses (such as BPI). Reordered the analyses
requests and early return if there are no loops. This is an NFC with
compile time improvement.

The same will be done in a follow-up patch for the loop vectorizer.

[nikic]

LG, this is basically the same thing FunctionToLoopPassAdaptor does.

[anna]

thanks for the quick review!

[fhahn]

Nice, LGTM!

llvm/lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp

IntersectUnsignedRange(ScalarEvolution &SE,
   // If the resulting range is empty, just return None.
   auto Ret = InductiveRangeCheck::Range(NewBegin, NewEnd);
   if (Ret.isEmpty(SE, /* IsSigned */ false))
     return None;
   return Ret;
 }
 
 PreservedAnalyses IRCEPass::run(Function &F, FunctionAnalysisManager &AM) {
-  auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
   auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
-  auto &BPI = AM.getResult<BranchProbabilityAnalysis>(F);
   LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
+  // There are no loops in the function. Return before computing other expensive
+  // analyses.
+  if (LI.empty())
+    return PreservedAnalyses::all();
+  auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
+  auto &BPI = AM.getResult<BranchProbabilityAnalysis>(F);
 
   // Get BFI analysis result on demand. Please note that modification of
   // CFG invalidates this analysis and we should handle it.
   auto getBFI = [&F, &AM ]()->BlockFrequencyInfo & {
     return AM.getResult<BlockFrequencyAnalysis>(F);
   };
   InductiveRangeCheckElimination IRCE(SE, &BPI, DT, LI, { getBFI });