[SPIR-V] Add legalize-addrspace-cast pass

[Keenuts]

This commit adds a new pass in the backend which propagates the
addrspace of the pointers down to the last use, making sure the
addrspace remains consistent, and thus stripping any addrspacecast.
This is required to lower LLVM-IR to logical SPIR-V, which does not
support generic pointers.

This is now required as HLSL emits several address spaces, and thus
addrspacecasts in some cases:

Example 1: resource access

%handle = tail call target("spirv.VulkanBuffer", ...)
%rptr = @llvm.spv.resource.getpointer(%handle, ...);
%cptr = addrspacecast ptr addrspace(11) %rptr to ptr
%fptr = load i32, ptr %cptr

Example 2: object methods

define void @objectmethod(ptr %this) {
}

define void @foo(ptr addrspace(11) %object) {
  call void @objectmethod(ptr addrspacecast(addrspace(11) %object to ptr));
}

This is waiting on #135794

[Naghasan]

Why not using the existing address space inference pass ? It looks redundant

[Keenuts]

Why not using the existing address space inference pass ? It looks redundant

That's a good question, and reason was I didn't knew this existed. I just tried, the llvm/test/CodeGen/SPIRV/pointers/pointer-addrspacecast.ll test pass but llvm/test/CodeGen/SPIRV/pointers/resource-addrspacecast.ll don't.

I need to look into why the passes fails to infer the resource address spaces.

[Keenuts]

Thanks a lot @Naghasan , looks like this is what we needed in the end. Opened #137766 and closing this one.