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

…reachable

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.

Differential Revision: https://reviews.llvm.org/D152434

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -1518,7 +1518,9 @@ Instruction *llvm::SplitBlockAndInsertIfThen(Value *Cond,
 
   if (LI) {
     if (Loop *L = LI->getLoopFor(Head)) {
-      L->addBasicBlockToLoop(ThenBlock, *LI);
+      // unreachable-terminated blocks cannot belong to any loop.
+      if (!Unreachable)
+        L->addBasicBlockToLoop(ThenBlock, *LI);
       L->addBasicBlockToLoop(Tail, *LI);
     }
   }

llvm/test/Transforms/SimpleLoopUnswitch/guards.ll

@@ -2,9 +2,6 @@
 ; 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) {

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

@@ -7,9 +7,6 @@ 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: