tcg/tcg-op: Document wswap_i64() byte pattern

Document wswap_i64(), added in commit 46be842
("tcg: Implement tcg_gen_{h,w}swap_{i32,i64}").

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230823145542.79633-8-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

tcg/tcg-op.c

@@ -1950,6 +1950,11 @@ void tcg_gen_hswap_i64(TCGv_i64 ret, TCGv_i64 arg)
     tcg_temp_free_i64(t1);
 }
 
+/*
+ * wswap_i64: Swap 32-bit words within a 64-bit value.
+ *
+ * Byte pattern: abcdefgh -> efghabcd
+ */
 void tcg_gen_wswap_i64(TCGv_i64 ret, TCGv_i64 arg)
 {
     /* Swapping 2 32-bit elements is a rotate. */