Zero-extend instructions in spike-dasm

...since we no longer rely on their being sign-extended.

spike_dasm/spike-dasm.cc

@@ -50,14 +50,10 @@ int main(int argc, char** argv)
         continue;
 
       char* endp;
-      int64_t bits = strtoull(&s[pos], &endp, 16);
+      insn_bits_t bits = strtoull(&s[pos], &endp, 16);
       if (*endp != ')')
         continue;
 
-      size_t nbits = 4 * (endp - &s[pos]);
-      if (nbits < 64)
-        bits = bits << (64 - nbits) >> (64 - nbits);
-
       string dis = disassembler->disassemble(bits);
       s = s.substr(0, start) + dis + s.substr(endp - &s[0] + 1);
       pos = start + dis.length();