[X86][MC] Fix wrong action when encoding enqcmd/enqcmds (#77571)

Mentioned in https://github.com/llvm/llvm-project/pull/77293,
enqcmd/enqcmds are special for its mem operand, like movdir64b(see
https://github.com/llvm/llvm-project/commit/4dd5e9c60efa9), 0x67 prefix
can not only modify its address size, so it's mem base and index reg
should be the same type as source reg.