[llvm] [X86][Gisel] add trivial arith tests for gisel x86-prelegalizer-combiner (PR #183544)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

tbh "trivial" feels like a dumping ground for tests that don't fit anwhere else.

I tend to prefer combine tests to be split by op type:
```
prelegalizer-combiner-add.mir
prelegalizer-combiner-sub.mir
etc.
```
or common fold pattern:
```
postlegalizer-combiner-identity.mir
```
postlegalizer-combiner-identity.mir already exists - most of these trivial folds can probably go in a prelegalizer-combiner-identity.mir equivalent?

https://github.com/llvm/llvm-project/pull/183544