[CIR][LLVMLowering] Add LLVM lowering for complex operations

[Lancern]

This PR adds LLVM lowering for the following operations related to complex numbers:

• cir.complex.create,
• cir.complex.real_ptr, and
• cir.complex.imag_ptr.

The LLVM IR generated for cir.complex.create is a bit ugly since it includes the insertvalue instruction, which typically is not generated in upstream CodeGen. Later we may need further CIR canonicalization passes to try folding cir.complex.create.

[bcardosolopes]

LGTM, will merge after the question below

The LLVM IR generated for cir.complex.create is a bit ugly since it includes the insertvalue instruction, which typically is not generated in upstream CodeGen. Later we may need further CIR canonicalization passes to try folding cir.complex.create.

Awesome! Is the insertvalue solely because we need better canonicalization at the CIR level? Can you please create an issue with the examples that need to be improved? Perhaps such canonicalization can be done in ComplexCreateOp::fold.

[Lancern]

Is the insertvalue solely because we need better canonicalization at the CIR level?

Yes, we currently cannot avoid this unless we could eliminate cir.complex.create entirely during canonicalization.

Can you please create an issue with the examples that need to be improved? Perhaps such canonicalization can be done in ComplexCreateOp::fold.

Created #726 to track this.