[llvm] [DAGCombiner] Add some very basic folds for ADDRSPACECAST (PR #127733)

[Matt Arsenault via llvm-commits]

================
@@ -16054,6 +16056,25 @@ SDValue DAGCombiner::visitBITCAST(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitADDRSPACECAST(SDNode *N) {
+  auto *ASCN1 = cast<AddrSpaceCastSDNode>(N);
+
+  if (auto *ASCN2 = dyn_cast<AddrSpaceCastSDNode>(ASCN1->getOperand(0))) {
+    assert(ASCN2->getDestAddressSpace() == ASCN1->getSrcAddressSpace());
+
+    // Fold asc[B -> A](asc[A -> B](x)) -> x
+    if (ASCN1->getDestAddressSpace() == ASCN2->getSrcAddressSpace())
+      return ASCN2->getOperand(0);
+
+    // Fold asc[B -> C](asc[A -> B](x)) -> asc[A -> C](x)
----------------
arsenm wrote:

> ASC(C->U->V) and ASC(C->W->V) will be distinctly different from ASC(C->V), as each will be pointing at a different window in V.

For the numeric pointer value, you may end up with a different bitpattern. At the ultimate access, through either path, there needs to be a single dereferenceable object at a single address in V.  It's possible we need the usage context implies UB on access to assume transitivity

https://github.com/llvm/llvm-project/pull/127733