Fix stack adjustment for x86 pop r16

[haileys]

Related to #4028

pop r16 is lifted incorrectly and adjusts the stack by the wrong size:

This PR changes the lifting to always pop the memory operand size and insert a LLIL_LOW_PART if the destination register size is different:

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[fuzyll]

Sorry for the delay on this.

I was a little confused when starting to review this because I didn't think x86-64 had an r16. I now understand that this is fixing lifting for things like segment registers where the destination is smaller than width of the stack pop. Not some weird general-purpose register thing. 😛

I verified that stuff like pop fs and pop gs appear improved while stuff like pop ax and pop word [rax] seem unaffected and the code itself seems like a clean change. I also verified that the change to push appears to not change anything and is just a refactor.

Thanks for the PR!