[llvm] [MC][x86] Fix missing check in MC binary expression (PR #75693)

[Chenyang Gao via llvm-commits]

cygao90 wrote:

@phoebewang Thank you for your review. I tested on gcc. gcc will also accept such instruction:
https://godbolt.org/z/er5zEhThY
but assembler will reject it:
https://godbolt.org/z/4EnPq7rjq
Perhaps we should leave it to llvm assembler
```
echo 'setb %al %rbx' | ./llvm-mc --filetype=obj -o test
<stdin>:1:1: error: expected relocatable expression
setb %al %rbx
^
```


https://github.com/llvm/llvm-project/pull/75693