[MemCpyOpt] Add stack move test with ret-only capture (NFC)

From:
#125880 (comment)

Author: nikic

llvm/test/Transforms/MemCpyOpt/stack-move.ll

@@ -1715,3 +1715,24 @@ else:                                             ; preds = %then, %entry
 
   uselistorder ptr %dest, { 1, 2, 0 }
 }
+
+declare ptr @captures_ret_only(ptr captures(ret: address, provenance))
+
+define i32 @test_ret_only_capture() {
+; CHECK-LABEL: define i32 @test_ret_only_capture() {
+; CHECK-NEXT:    [[A:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    [[B:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    store i32 0, ptr [[A]], align 4
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p0.i64(ptr [[B]], ptr [[A]], i64 4, i1 false)
+; CHECK-NEXT:    call void @captures_ret_only(ptr [[B]])
+; CHECK-NEXT:    [[V:%.*]] = load i32, ptr [[A]], align 4
+; CHECK-NEXT:    ret i32 [[V]]
+;
+  %a = alloca i32
+  %b = alloca i32
+  store i32 0, ptr %a
+  call void @llvm.memcpy(ptr %b, ptr %a, i64 4, i1 false)
+  call void @captures_ret_only(ptr %b)
+  %v = load i32, ptr %a
+  ret i32 %v
+}