target-arm: fix bug in translation of REVSH

The translation of REVSH shifted the low byte 8 steps left before performing
an 8-bit sign extend, causing this part of the expression to alwas be 0.
Reported-by: NJohan Bengtsson <teofrastius@gmail.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

target-arm/translate.c

@@ -250,13 +250,9 @@ static void gen_rev16(TCGv var)
 /* Byteswap low halfword and sign extend.  */
 static void gen_revsh(TCGv var)
 {
-    TCGv tmp = new_tmp();
-    tcg_gen_shri_i32(tmp, var, 8);
-    tcg_gen_andi_i32(tmp, tmp, 0x00ff);
-    tcg_gen_shli_i32(var, var, 8);
-    tcg_gen_ext8s_i32(var, var);
-    tcg_gen_or_i32(var, var, tmp);
-    dead_tmp(tmp);
+    tcg_gen_ext16u_i32(var, var);
+    tcg_gen_bswap16_i32(var, var);
+    tcg_gen_ext16s_i32(var, var);
 }
 
 /* Unsigned bitfield extract.  */