x86: adjust struct instr_info field types

Now that this lives on the stack, let's have it be a little less
wasteful in terms of space. Switch boolean fields to "bool" (also when
this doesn't change their size) and also limit the widths of "rex",
"rex_used", "op_ad", and "op_index". Do a little bit of re-ordering as
well to limit the number of padding holes.