[mlir] Ensure fmaximum/fminimum is defined in mlir/Dialect/LLVMIR/LLVMEnums.td (#138198)
Fix a compile error (with LLVM_ENABLE_WERROR=On) when building
`bin/flang`:
```
enumeration values 'FMaximum' and 'FMinimum' not handled in switch
```
caused by adding new LLVM IR instructions in #137701.
This wasn't picked up by the auto CI builders on GitHub, so I hadn't
realised until @kazutakahirata notified me about it.