[mlir][spirv] Make EntryPointABIAttr.local_size optional

* It doesn't required by OpenCL/Intel Level Zero and can be set programmatically.
* Add GPU to spirv lowering in case when attribute is not present.
* Set higher benefit to WorkGroupSizeConversion pattern so it will always try to lower first from the attribute.

Differential Revision: https://reviews.llvm.org/D120399