(#373) Fix pi = 4 in amd64 assembly-gen

[herrhotzenplotz]

• Clean out rax before doing the comparison to sweep out the upper
  bits as only the lower 16 bits get set (Thanks to abridgewater)
• Fix cleaning xmm0 (Thanks to kolumbetko)

[zhiayang]

if you want to save 1 byte for each xor, you can use xor eax, eax as well :D

[herrhotzenplotz]

if you want to save 1 byte for each xor, you can use xor eax, eax as well :D

<insert rant on x86/>
but then I'd go through the entire generator and change rax to eax to make it consistent. I guess that is another issue.

[zhiayang]

it's fine to leave it like this, more consistent anyway.

[rexim]

@herrhotzenplotz @kolumb @abridgewater looks goode to me! 👍 Thank you for your contribution!