[CIR][WIP] Add ABI lowering pass

[Lancern]

This PR attempts to add a new pass cir-abi-lowering to the CIR dialect. This pass runs before the CallConvLowering pass, and it expands all ABI-dependent types and operations inside a function to their ABI-independent equivalences according to the ABI specification.

The patch also moves the lowering code of the following types and operations from the LLVM lowering conversion to the new pass:

• The pointer-to-data-member type cir.data_member;
• The pointer-to-member-function type cir.method;
• All operations working on operands of the above types.

[Lancern]

The direct motivation for this new pass is the proper CallConvLowering of the !cir.method type. Currently, this type is lowered during LLVM lowering, which is too late to achieve a proper lowering. Consider the following CIR code:

cir.func @test(%arg0: !cir.method) {
  // ...
}

Following the current lowering approach, which keeps !cir.method until we arrive at LLVM lowering, we would eventually get the following LLVM IR:

define dso_local @test({ i64, i64 } %0) {
  ; ...
}

But we actually expect the following LLVM IR (note the differences on the function signatures):

define dso_local @test(i64 %0, i64 %1) {
  ; ...
}

To achieve this, I have 3 choices:

1. Teach the CallConvLowering pass about the !cir.method type.
2. Move the lowering of !cir.method to the LoweringPrepare pass, which runs before CallConvLowering.
3. Add a new pass before CallConvLowering that lowers !cir.method to !cir.struct.

At the beginning I thought option 1 would be the easiest way. But as I dig through the rabbit hole I found some tricky stuff behind the scene. The problem comes from the CodeGen of function prologue and epilogue. In the prologue, each argument is assigned a stack slot and stored there. For an argument of type !cir.method, after CallConvLowering it would expands into two arguments of type !s64i. Thus in the function prologue I would have to come up a way to store two !s64i values into the stack slot allocated for a !cir.method value, which is tricky. Similar problems also exist in the epilogue.

The problem of option 3 is that the LoweringPrepare pass is not a conversion pass, which could be really tricky if you want to do type conversion stuff in it. In my case I have to convert every appearances of !cir.method to !cir.struct and this kind of job is better suited for a conversion pass.

Anyway, this PR is still very incomplete and under construction, I'd like to hear some early comments about this from the community.