64-bit move can be made shorter

[www.agner.org]

mov rax,1
is coded with a 32-bit sign-extended constant. The zero extended form is two bytes shorter. Note that mov eax,1 is the same as mov rax,1 because of implicit zero-extension.

mov rcx,0xaaaaaaaa
is coded with a full 64-bit constant. The zero-extended version is 5 bytes shorter.

The zero-extended version version is optimal when the constant is in the range
0 <= n < 2³².
The sign-extended version is optimal in the range
-2³¹ <= n < 0