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>

Author: 4a6f656c

test/codegen/shift.go

@@ -661,7 +661,7 @@ func rsh64to32(v int64) int32 {
 	x := int32(v)
 	// riscv64:"MOVW"
 	if x > 8 {
-		// riscv64:"SRAIW" -"MOVW" -"SLLI"
+		// riscv64:"SRLIW" -"MOVW" -"SLLI"
 		x >>= 2
 	}
 	return x
@@ -671,7 +671,7 @@ func rsh64to16(v int64) int16 {
 	x := int16(v)
 	// riscv64:"MOVH"
 	if x > 8 {
-		// riscv64:"SLLI" "SRAI"
+		// riscv64:"SLLI" "SRLI"
 		x >>= 2
 	}
 	return x
@@ -681,7 +681,7 @@ func rsh64to8(v int64) int8 {
 	x := int8(v)
 	// riscv64:"MOVB"
 	if x > 8 {
-		// riscv64:"SLLI" "SRAI"
+		// riscv64:"SLLI" "SRLI"
 		x >>= 2
 	}
 	return x