Generate 0xFFFFFFFF instead of 0 for rs1 field in make_go

The default rs1 is 0, but this may make encoding process hard to
distinguish the real rs1 of an instruction.

We set it to 0xFFFFFFFF instead.

parse.py

@@ -966,9 +966,10 @@ def make_go(instr_dict):
     instr_str = ''
     for i in instr_dict:
         enc_match = int(instr_dict[i]['match'],0)
+        enc_encoding = instr_dict[i]['encoding']
         opcode = (enc_match >> 0) & ((1<<7)-1)
         funct3 = (enc_match >> 12) & ((1<<3)-1)
-        rs1 = (enc_match >> 15) & ((1<<5)-1)
+        rs1 = (enc_match >> 15) & ((1<<5)-1) if enc_encoding[12:17:] != "-----" else 0xFFFFFFFF
         rs2 = (enc_match >> 20) & ((1<<5)-1)
         csr = (enc_match >> 20) & ((1<<12)-1)
         funct7 = (enc_match >> 25) & ((1<<7)-1)