[llvm] MC: Make POP imply pointer-size operands, as with PUSH (PR #114533)

[Fangrui Song via llvm-commits]

MaskRay wrote:

LGTM. This fixes

`clang -masm=intel -c -x assembler - <<< 'pop [eax]'` (GCC with gas works)

https://github.com/llvm/llvm-project/pull/114533