[CIR][CIRGen] Add support for global named registers

[566hub]

Realize RG and GET_RG op which are copied from cir.Global
op. The concept is that the same asm label means a unique abstract memory
address with u64i type, and get_rg will return ptr. We could use
cir.cast(bitcast) to transform ptr to ptr.

[bcardosolopes]

Thanks for working on this, it's a nice feature addition!

We can achieve the same by re-using existing ClangIR components. The suggested approach:

• Create/use a regular global.
• Use regular cir.get_global to retrieve the address.
• Add a new attribute #cir.asm_reg that holds a StringAttr. This should be used to initialize the global.
• Add a isGlobalRegister method to cir.global's extraClassDeclaration, which checks if the global has an initializer and whether that is a #cir.asm_reg.
• Add support in LowerToLLVM.cpp to materialize cir.get_globals into metadata !<whatever>. cir.{load,store} referring to these globals should be converted to the proper intrinsic calls.
• Use a more complete usecase that exercise both intrinsics: https://godbolt.org/z/deGs17rox
• Use clang/test/CIR/CodeGen/abstract-cond.c as an example for testing both CIR and LLVM output in the same test.

[bcardosolopes]

Thanks for trying to improve this! We don't eagerly lower things to LLVM right away though, this is done in LowerToLLVM.cpp.