fix(asm): bound the riscv64 shift immediate at the instruction width

SLLI $64 assembled with the amount silently masked into the six-bit
field where the toolchain rejects it, and the word forms took 0-63 where
they take 0-31.  Both families now validate against their own width and
the check reads the immediate at full width, so a value the source
spelled beyond int32 cannot wrap into the range; the boundary is pinned
in a test.

Assisted-by: GLM 5.3 Flash

asm/riscv_encode_test.go

@@ -1389,3 +1389,32 @@ func TestRISCV_rawDataRange(t *testing.T) {
 		}
 	})
 }
+
+// TestRISCV_shiftImmediateRange pins the shift immediate at the toolchain's
+// validation boundary: 0-63 on the doubleword forms, 0-31 on the word forms,
+// values beyond rejected on sight rather than masked into the field.
+func TestRISCV_shiftImmediateRange(t *testing.T) {
+	asmOne := func(t *testing.T, stmt string) error {
+		t.Helper()
+		fn := firstTextRISCV(t, "#include \"textflag.h\"\nTEXT ·s(SB), NOSPLIT, $0\n\t"+stmt+"\n\tRET\n")
+		_, _, _, _, _, _, err := assembleRISCV(fn)
+		return err
+	}
+	for _, s := range []string{
+		"SLLI $63, X5, X6", "SLLI $0, X5, X6", "SRLI $63, X5", "SRAI $1, X5, X6",
+		"SLLIW $31, X5, X6", "SRLIW $31, X5", "SRAIW $1, X5, X6",
+	} {
+		if err := asmOne(t, s); err != nil {
+			t.Errorf("%s must assemble: %v", s, err)
+		}
+	}
+	for _, s := range []string{
+		"SLLI $64, X5, X6", "SLLI $-1, X5", "SLLI $0x100000000, X5, X6",
+		"SRLI $64, X5", "SRAI $-1, X5, X6",
+		"SLLIW $32, X5, X6", "SRLIW $-1, X5", "SRAIW $32, X5, X6",
+	} {
+		if err := asmOne(t, s); err == nil {
+			t.Errorf("%s must be rejected, as go tool asm rejects it", s)
+		}
+	}
+}