[llvm][amdgpu] Handle indirect refs to LDS GVs during LDS lowering (#124089)

Fixes #123800

Extends LDS lowering by allowing it to discover transitive
indirect/escpaing references to LDS GVs.

For example, given the following input:
```llvm
@lds_item_to_indirectly_load = internal addrspace(3) global ptr undef, align 8

%store_type = type { i32, ptr }
@place_to_store_indirect_caller = internal addrspace(3) global %store_type undef, align 8

define amdgpu_kernel void @offloading_kernel() {
  store ptr @indirectly_load_lds, ptr addrspace(3) getelementptr inbounds nuw (i8, ptr addrspace(3) @place_to_store_indirect_caller, i32 0), align 8
  call void @call_unknown()
  ret void
}

define void @call_unknown() {
  %1 = alloca ptr, align 8
  %2 = call i32 %1()
  ret void
}

define void @indirectly_load_lds() {
  call void @directly_load_lds()
  ret void
}

define void @directly_load_lds() {
  %2 = load ptr, ptr addrspace(3) @lds_item_to_indirectly_load, align 8
  ret void
}

```

With the above input, prior to this patch, LDS lowering failed to lower
the reference to `@lds_item_to_indirectly_load` because:
1. it is indirectly called by a function whose address is taken in the
kernel.
2. we did not check if the kernel indirectly makes any calls to unknown
functions (we only checked the direct calls).

Co-authored-by: Jon Chesterfield <jonathan.chesterfield@amd.com>