[llvm] [TableGen] Make sure BitWidth is set evem if all instructions are Pseudo or TargetOpcode (PR #124071)

[Farzon Lotfi via llvm-commits]

farzonl wrote:

> What is the use case for a target with only TargetOpcode and Pseudos?

I'm experimenting with moving the DirectX backend to GlobalIsel. Since DXIL is just LLVMIR  3.7 Most everything we need is covered via TargetOpcodes.

https://github.com/llvm/llvm-project/pull/124071