x86: simplify / consolidate check_{word,long,qword}_reg()

These run after template matching. Therefore operands are already known
to match the template in use. With the loop bodies skipping anything not
a GPR in the actual operands, there's therefore no need to check the
template's operand type for permitting Reg or Accum.

At the same time bring the three functions in sync for the "byte" part
of the logic, as far as checking the template for other sizes (qword
specifically) goes. Plus drop a stale comment from check_qword_reg(),
when all three are now behaving the same in this regard.