[CIR][LowerToLLVM][NFC] Refactor GlobalOpLowering for better readability and maintainability

[seven-mile]

As noted in this comment, the nested if-arms in GlobalOpLowering are somewhat confusing and error-prone. This PR simplifies the logic into more straightforward components.

Since LLVM's GlobalOp accepts two types of initializers (either an initializer value or an initializer region), we've extracted the decision logic into a separate function called lowerInitializer. This function takes two inout arguments: mlir::Attribute &init (for the attribute value) and bool useInitializerRegion (as the decision indicator). All code paths then converge at a common epilogue that handles the operation rewriting.

The previous implementation for lowering DataMemberAttr initializers relied on recursion between MLIR rewrite calls, which made the control flow somewhat opaque. The new version makes this explicit by using a clear self-recursive pattern within lowerInitializer.

[bcardosolopes]

The previous implementation for lowering DataMemberAttr initializers relied on recursion between MLIR rewrite calls

Perhaps at some point we should change this to some iteration algorithm instead.

[bcardosolopes]

Awesome, thanks for the cleanup!