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

[Alex MacLean 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)
----------------
AlexMaclean wrote:

@nikic Could you please help clarify the semantics here? In practice, the fact that we're treating ASC as transitive in IC makes me think no target has defined address spaces as Artem is describing, meaning it should be safe here too.

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