[llvm] [MC][x86] Allow non-MCTargetExpr RHS when the LHS of a MCBinaryExpr is MCTargetExpr (PR #75693)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Can someone with write access merge this PR if no further change is required? Thanks.

We should add a test `.ifdef 1 == var_xdata` as well and ensure there is an EOL at the end of the file. I merged the new test file into `register-assignment.s` in a22c8efad733cf3407a93387a6a9c743659dc74c.

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