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

[Fangrui Song via llvm-commits]

MaskRay wrote:

I fixed `MCBinaryExpr::NE` and extended `llvm/test/MC/X86/register-assignment.s`, but did not think much about non-MCTargetExpr `RHS`.

I think this patch is correct but we need a test to `llvm/test/MC/X86/register-assignment.s`. It can be `.if var_cata == 1 ...`

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