[ValueTracking] Return true for AddrSpaceCast in canCreateUndefOrPoison (#144686)

In our downstream GPU target, following IR is valid before instcombine
although the second addrspacecast causes UB.
  define i1 @test(ptr addrspace(1) noundef %v) {
    %0 = addrspacecast ptr addrspace(1) %v to ptr addrspace(4)
    %1 = call i32 @llvm.xxxx.isaddr.shared(ptr addrspace(4) %0)
    %2 = icmp eq i32 %1, 0
    %3 = addrspacecast ptr addrspace(4) %0 to ptr addrspace(3)
    %4 = select i1 %2, ptr addrspace(3) null, ptr addrspace(3) %3
    %5 = icmp eq ptr addrspace(3) %4, null
    ret i1 %5
  }
We have a custom optimization that replaces invalid addrspacecast with
poison, and IR is still valid since `select` stops poison propagation.

However, instcombine pass optimizes `select` to `or`:
    %0 = addrspacecast ptr addrspace(1) %v to ptr addrspace(4)
    %1 = call i32 @llvm.xxxx.isaddr.shared(ptr addrspace(4) %0)
    %2 = icmp eq i32 %1, 0
    %3 = addrspacecast ptr addrspace(1) %v to ptr addrspace(3)
    %4 = icmp eq ptr addrspace(3) %3, null
    %5 = or i1 %2, %4
    ret i1 %5
The transform is invalid for our target.

---------

Co-authored-by: Nikita Popov <github@npopov.com>

llvm/docs/LangRef.rst

@@ -12627,6 +12627,9 @@ result pointer is dereferenceable, the cast is assumed to be
 reversible (i.e. casting the result back to the original address space
 should yield the original bit pattern).
 
+Which address space casts are supported depends on the target. Unsupported
+address space casts return :ref:`poison <poisonvalues>`.
+
 Example:
 """"""""