[X86] Refine X86DAGToDAGISel::isSExtAbsoluteSymbolRef() (#76191)

We just need to check if the global is large or not.

In the kernel code model, globals are in the negative 2GB of the address
space, so globals can be a sign extended 32-bit immediate.

In other code models, small globals are in the low 2GB of the address
space, so sign extending them is equivalent to zero extending them.