test/codegen: fix shift tests on riscv64

test/codegen: fix shift tests on riscv64

These were broken by CL 721206, which changes Rsh to RshU for
positive inputs.

Change-Id: I9e38c3c428fb8aeb70cf51e7e76f4711c864f027
Reviewed-on: https://go-review.googlesource.com/c/go/+/723340
Reviewed-by: Meng Zhuo <mengzhuo1203@gmail.com>
Reviewed-by: Mark Ryan <markdryan@rivosinc.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>