Subzero: Recognize single-block loops during loop depth analysis.

tests_lit/llvm2ice_tests/loop-nest-depth.ll

Index: tests_lit/llvm2ice_tests/loop-nest-depth.ll
diff --git a/tests_lit/llvm2ice_tests/loop-nest-depth.ll b/tests_lit/llvm2ice_tests/loop-nest-depth.ll
index 8baf4350a32d6d05ff9aea21efac36d1db772ec3..a899e50c0f263dbaf43d7f5f7b9a9ea8c512adca 100644
--- a/tests_lit/llvm2ice_tests/loop-nest-depth.ll
+++ b/tests_lit/llvm2ice_tests/loop-nest-depth.ll
@@ -279,3 +279,28 @@ out:
 ; CHECK-NEXT: out:
 ; CHECK-NEXT: LoopNestDepth = 0
 ; CHECK-LABEL: Before RMW
+
+define internal void @test_single_block_loop(i32 %count) {
+entry:
+  br label %body
+body:
+;  %i = phi i32 [ 0, %entry ], [ %inc, %body ]
+; A normal loop would have a phi instruction like above for the induction
+; variable, but that may introduce new basic blocks due to phi edge splitting,
+; so we use an alternative definition for %i to make the test more clear.
+  %i = add i32 %count, 1
+  %inc = add i32 %i, 1
+  %cmp = icmp slt i32 %inc, %count
+  br i1 %cmp, label %body, label %exit
+exit:
+  ret void
+}
+
+; CHECK-LABEL: After loop nest depth analysis
+; CHECK-NEXT: entry:
+; CHECK-NEXT: LoopNestDepth = 0
+; CHECK-NEXT: body:
+; CHECK-NEXT: LoopNestDepth = 1
+; CHECK-NEXT: exit:
+; CHECK-NEXT: LoopNestDepth = 0
+; CHECK-LABEL: Before RMW