i#2491 ARM unallocated encodings: Better handling of undecoded instrs.

In check_encode_decode_consistency(), do not try to reencode an
OP_UNDECODED instruction.

In instr_encode_arch(), use instr->opcode, not instr_get_opcode(instr),
as the latter can call the decoder, which is unhelpful when we are
trying to encode.

Fixes #2491

Change-Id: I66c42dc87268f1722eae4600b364026896796fc8

core/arch/arm/decode.c

@@ -2893,8 +2893,12 @@ check_encode_decode_consistency(dcontext_t *dcontext, instrlist_t *ilist)
         byte buf[THUMB_LONG_INSTR_SIZE];
         instr_t tmp;
         byte *pc, *npc;
-        app_pc addr = instr_get_raw_bits(check);
-        int check_len = instr_length(dcontext, check);
+        app_pc addr;
+        int check_len;
+        if (check->opcode == OP_UNDECODED)
+            continue;
+        addr = instr_get_raw_bits(check);
+        check_len = instr_length(dcontext, check);
         instr_set_raw_bits_valid(check, false);
         pc = instr_encode_to_copy(dcontext, check, buf, addr);
         instr_init(dcontext, &tmp);

core/arch/arm/encode.c

@@ -2766,7 +2766,7 @@ instr_encode_arch(dcontext_t *dcontext, instr_t *instr, byte *copy_pc, byte *fin
     }
 
     decode_info_init_for_instr(&di, instr);
-    di.opcode = instr_get_opcode(instr);
+    di.opcode = instr->opcode;
     di.check_reachable = check_reachable;
     di.start_pc = copy_pc;
     di.final_pc = final_pc;