[LV] Add test showing debug output for loops with uncountable BTCs.

Currently we print an early-exit related related debug message, even
though there's no early exit.

llvm/test/Transforms/LoopVectorize/uncountable-single-exit-loops.ll

@@ -0,0 +1,52 @@
+; REQUIRES: asserts
+; RUN: opt -p loop-vectorize -debug %s 2>&1 | FileCheck %s
+
+
+; CHECK-LABEL: LV: Checking a loop in 'latch_exit_cannot_compute_btc_due_to_step'
+; CHECK: 	   LV: Did not find one integer induction var.
+; CHECK-NEXT:  LV: Not vectorizing: Early exit is not the latch predecessor.
+; CHECK-NEXT:  LV: Interleaving disabled by the pass manager
+; CHECK-NEXT:  LV: Not vectorizing: Cannot prove legality.
+
+; CHECK-LABEL: LV: Checking a loop in 'header_exit_cannot_compute_btc_due_to_step'
+; CHECK:       LV: Found an induction variable.
+; CHECK-NEXT:  LV: Did not find one integer induction var.
+; CHECK-NEXT:  LV: Not vectorizing: Cannot determine exact exit count for latch block.
+; CHECK-NEXT:  LV: Interleaving disabled by the pass manager
+; CHECK-NEXT:  LV: Not vectorizing: Cannot prove legality.
+
+; CHECK-NOT: vector.body
+define void @latch_exit_cannot_compute_btc_due_to_step(ptr %dst, i64 %step) {
+entry:
+  br label %loop
+
+loop:                                   ; preds = %loop, %for.cond.us
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
+  %iv.next = add i64 %iv, %step
+  %gep = getelementptr i8, ptr %dst, i64 %iv
+  store i8 0, ptr %gep, align 1
+  %ec = icmp eq i64 %iv.next, 1000
+  br i1 %ec, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @header_exit_cannot_compute_btc_due_to_step(ptr %dst, i64 %step) {
+entry:
+  br label %loop.header
+
+loop.header:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop.latch ]
+  %iv.next = add i64 %iv, %step
+  %ec = icmp eq i64 %iv.next, 1000
+  br i1 %ec, label %loop.latch, label %exit
+
+loop.latch:
+  %gep = getelementptr i8, ptr %dst, i64 %iv
+  store i8 0, ptr %gep, align 1
+  br label %loop.header
+
+exit:
+  ret void
+}