Remove obsolete check for TTSOpsVirtual from slot_compile_deform

Hi hackers,

While exploring the JIT support for tuple deforming process, I noticed that one check for TTSOpsVirtual in slot_compile_deform is obsolete. Since virtual tuples never need deforming and there's an assertion in llvm_compile_expr[1]. I simply replace it with an assertion in slot_compile_deform. Patch is attached.

[1] https://github.com/postgres/postgres/blob/0c60e8ba80e03491b028204a19a9dca6d216df91/src/backend/jit/llvm/llvmjit_expr.c#L322