[SCEV] Add test for unnecessary umax in BECount (NFC)

Author: nikic

llvm/test/Analysis/ScalarEvolution/trip-count.ll

@@ -123,3 +123,28 @@ loop:
 leave:
   ret void
 }
+
+define void @non_zero_from_loop_guard(i16 %n) {
+; CHECK-LABEL: 'non_zero_from_loop_guard'
+; CHECK-NEXT:  Determining loop execution counts for: @non_zero_from_loop_guard
+; CHECK-NEXT:  Loop %loop: backedge-taken count is (-1 + (1 umax (%n /u 2)))<nsw>
+; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is 32766
+; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is (-1 + (1 umax (%n /u 2)))<nsw>
+; CHECK-NEXT:  Loop %loop: Predicated backedge-taken count is (-1 + (1 umax (%n /u 2)))<nsw>
+; CHECK-NEXT:   Predicates:
+; CHECK-NEXT:  Loop %loop: Trip multiple is 1
+;
+entry:
+  %shr = lshr i16 %n, 1
+  %precond = icmp ult i16 %n, 2
+  br i1 %precond, label %exit, label %loop
+
+loop:
+  %iv = phi i16 [ %inc, %loop ], [ 0, %entry ]
+  %inc = add nuw nsw i16 %iv, 1
+  %cmp = icmp ult i16 %inc, %shr
+  br i1 %cmp, label %loop, label %exit
+
+exit:
+  ret void
+}