[CIR] CallConvLowering for X86 aggregate

[AdUhTkJm]

This deals with some x86 aggregate types for CallConvLowering pass.

Suppose we have a simple struct like this.

struct dim3 { int x, y, z; };

It can be coerced into

struct dim3_ { uint64_t xy; int z; };

And for a function that receives it as an argument, OG does the following transformation for x86:

void f(dim3 arg) { /* Before */ }
void f(uint64_t xy, int z) { /* After */ }

Now this transformation is implemented in the CallConvLowering pass of CIR.