[LoopInterchange] Check number of latch successors before accessing them.

[fhahn]

Summary

In cases where the OuterMostLoopLatchBI only has a single successor,
accessing the second successor will fail.

This fixes a failure when building the test-suite with loop-interchange
enabled.

[fhahn]

ping.

[karthikthecool]

Looks good to me.
Thanks

lib/Transforms/Scalar/LoopInterchange.cpp

 #ifdef DUMP_DEP_MATRICIES
     DEBUG(dbgs() << "Dependence before interchange\n");
     printDepMatrix(DependencyMatrix);
 #endif
 
     BasicBlock *OuterMostLoopLatch = OuterMostLoop->getLoopLatch();
     BranchInst *OuterMostLoopLatchBI =
         dyn_cast<BranchInst>(OuterMostLoopLatch->getTerminator());
-    if (!OuterMostLoopLatchBI)
+    if (!OuterMostLoopLatchBI || OuterMostLoopLatchBI->getNumSuccessors() != 2)
       return false;
 
     // Since we currently do not handle LCSSA PHI's any failure in loop
     // condition will now branch to LoopNestExit.
     // TODO: This should be removed once we handle LCSSA PHI nodes.
 
     // Get the Outermost loop exit.
     BasicBlock *LoopNestExit;

test/Transforms/LoopInterchange/interchange-latch-no-exit.ll

+; RUN: opt < %s -loop-interchange -S | FileCheck %s
+
+; BB latch1 is the loop latch, but does not exit the loop.
+define void @foo() {
+entry:
+  %dest = alloca i16*, align 8
+  br label %header1
+
+header1:
+  %0 = phi i16* [ %2, %latch1 ], [ undef, %entry ]
+  br i1 false, label %inner, label %loopexit
+
+inner:
+  br i1 undef, label %inner.ph, label %latch1
+
+inner.ph:
+  br label %inner.body
+
+inner.body:
+  %1 = load i16, i16* %0, align 2
+  store i16* inttoptr (i64 2 to i16*), i16** %dest, align 8
+  br i1 false, label %inner.body, label %inner.loopexit
+
+inner.loopexit:
+  br label %latch1
+
+latch1:
+  %2 = phi i16* [ %0, %inner ], [ undef, %inner.loopexit ]
+  br label %header1
+
+loopexit:                                         ; preds = %header1
+  unreachable
+}
+
+; CHECK-LABEL: inner.body:
+; CHECK: br i1 false, label %inner.body, label %inner.loopexit
+; CHECK: latch1:
+; CHECK-NEXT: %2 = phi i16* [ %0, %inner ], [ undef, %inner.loopexit ]
+; CHECK-NEXT: br label %header1
+