[LoopLoadElim] Fix crash by always checking simplify form

[mkazantsev]

Summary

Loop simplify form should always be checked because logic of
propagateStoredValueToLoadUsers relies on it (in particular, it requires preheader).

[fedor.sergeev]

LGTM, please give others at least 24h to respond.

[fhahn]

LGTM.

llvm/lib/Transforms/Scalar/LoopLoadElimination.cpp

bool processLoop() {
     }
 
     if (LAI.getPSE().getUnionPredicate().getComplexity() >
         LoadElimSCEVCheckThreshold) {
       LLVM_DEBUG(dbgs() << "Too many SCEV run-time checks needed.\n");
       return false;
     }
 
+    if (!L->isLoopSimplifyForm()) {
+      LLVM_DEBUG(dbgs() << "Loop is not is loop-simplify form");
+      return false;
+    }
+
     if (!Checks.empty() || !LAI.getPSE().getUnionPredicate().isAlwaysTrue()) {
       if (LAI.hasConvergentOp()) {
         LLVM_DEBUG(dbgs() << "Versioning is needed but not allowed with "
                              "convergent calls\n");
         return false;
       }
 
       auto *HeaderBB = L->getHeader();
       auto *F = HeaderBB->getParent();
       bool OptForSize = F->hasOptSize() ||
                         llvm::shouldOptimizeForSize(HeaderBB, PSI, BFI,
                                                     PGSOQueryType::IRPass);
       if (OptForSize) {
         LLVM_DEBUG(
             dbgs() << "Versioning is needed but not allowed when optimizing "
                       "for size.\n");
         return false;
       }
 
-      if (!L->isLoopSimplifyForm()) {
-        LLVM_DEBUG(dbgs() << "Loop is not is loop-simplify form");
-        return false;
-      }
-
       // Point of no-return, start the transformation.  First, version the loop
       // if necessary.
 
       LoopVersioning LV(LAI, L, LI, DT, PSE.getSE(), false);
       LV.setAliasChecks(std::move(Checks));
       LV.setSCEVChecks(LAI.getPSE().getUnionPredicate());
       LV.versionLoop();
     }

llvm/test/Transforms/LoopLoadElim/new-pm-preheader-problem.ll

-; XFAIL: *
 ; RUN: opt -passes=loop-load-elim -S < %s | FileCheck %s
-; This test demonstrates an assertion failure due to missing preheader in new PM.
-; REQUIRES: asserts
+; Make sure it doesn't crash in new pass manager due to missing preheader.
 target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
 
 define void @test(i32* noalias nocapture %A, i32* noalias nocapture readonly %B, i64 %N, i1 %C) {
 entry:
   br i1 %C, label %for.body, label %for.end
 
 ; CHECK: test
 for.body: