[BBUtils] Don't add 'then' block to a loop if it's terminated with unreachable

[dmakogon]

Summary

SplitBlockAndInsertIfThen utility creates two new blocks, they're called ThenBlock and Tail (true and false destinations of a conditional branch correspondingly). The function has a bool parameter Unreachable, and if it's set, then ThenBlock is terminated with an unreachable.
At the end of the function the new blocks are added to the loop of the split block. However, in case ThenBlock is terminated with an unreachable, it cannot belong to any loop.

The affected tests crashed in SimpleLoopUnswitch when it was turning guards into branches during LoopInfo verification which I added before this patch.

[nikic]

LGTM

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
   HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
   ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
 
   if (DTU)
     DTU->applyUpdates(Updates);
 
   if (LI) {
     if (Loop *L = LI->getLoopFor(Head)) {
+      // unreachable-terminated blocks cannot belong to any loop.
+      if (!Unreachable)
         L->addBasicBlockToLoop(ThenBlock, *LI);
       L->addBasicBlockToLoop(Tail, *LI);
     }
   }
 
   return CheckTerm;
 }
 
 void llvm::SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,

llvm/test/Transforms/SimpleLoopUnswitch/guards.ll

 ; RUN: opt -passes='loop(simple-loop-unswitch<nontrivial>),verify<loops>' -simple-loop-unswitch-guards -S < %s | FileCheck %s
 ; RUN: opt -passes='simple-loop-unswitch<nontrivial>' -simple-loop-unswitch-guards -S < %s | FileCheck %s
 ; RUN: opt -passes='loop-mssa(simple-loop-unswitch<nontrivial>),verify<loops>' -simple-loop-unswitch-guards  -verify-memoryssa -verify-loop-info -S < %s | FileCheck %s
 
-; XFAIL: *
-; REQUIRES: asserts
-
 declare void @llvm.experimental.guard(i1, ...)
 
 define void @test_simple_case(i1 %cond, i32 %N) {
 ; CHECK-LABEL: @test_simple_case(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 [[COND:%.*]], label [[ENTRY_SPLIT_US:%.*]], label [[ENTRY_SPLIT:%.*]]
 ; CHECK:       entry.split.us:
 ; CHECK-NEXT:    br label [[LOOP_US:%.*]]

llvm/test/Transforms/SimpleLoopUnswitch/nontrivial-unswitch-skip-selects-in-guards.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -passes='loop(simple-loop-unswitch<nontrivial>),verify<loops>' -S < %s | FileCheck %s
 ; RUN: opt -passes='loop-mssa(simple-loop-unswitch<nontrivial>),verify<loops>' -S < %s | FileCheck %s
 ; RUN: opt -passes='simple-loop-unswitch<nontrivial>' -verify-memoryssa -verify-loop-info -S < %s | FileCheck %s
 
 declare ptr @pluto()
 declare void @llvm.experimental.guard(i1, ...)
 declare void @widget()
 
-; XFAIL: *
-; REQUIRES: asserts
-
 define void @foo(ptr addrspace(1) %arg, i64 %arg1) personality ptr @pluto {
 ; CHECK-LABEL: @foo(
 ; CHECK-NEXT:  bb:
 ; CHECK-NEXT:    [[TMP:%.*]] = icmp slt i32 poison, 570
 ; CHECK-NEXT:    [[TMP2:%.*]] = select i1 [[TMP]], i1 true, i1 false
 ; CHECK-NEXT:    br i1 [[TMP]], label [[BB_SPLIT_US:%.*]], label [[BB_SPLIT:%.*]]
 ; CHECK:       bb.split.us:
 ; CHECK-NEXT:    br label [[BB3_US:%.*]]