[llvm] [SDAG] Preserve InBounds in DAGCombines (PR #165424)

[Alexander Richardson via llvm-commits]

================
@@ -2715,6 +2715,12 @@ SDValue DAGCombiner::visitPTRADD(SDNode *N) {
           (N->getFlags() & N0->getFlags()) & SDNodeFlags::NoUnsignedWrap;
       SDValue Add = DAG.getNode(ISD::ADD, DL, IntVT, {Y, Z}, Flags);
       AddToWorklist(Add.getNode());
+      // We can't set InBounds even if both original ptradds were InBounds and
+      // NUW: SDAG usually represents pointers as integers, therefore, the
+      // matched pattern behaves as if it had implicit casts:
+      //   (ptradd inbounds (inttoptr (ptrtoint (ptradd inbounds x, y))), z)
+      // The outer inbounds ptradd might therefore rely on a provenance that x
+      // does not have.
----------------
arichardson wrote:

Now that we have the c64/c128 value type upstream this could be changed to check PtrVT.isInteger()/PtrVT.isCheriCapability(), but I am also fine with deferring this until the real codegen support has landed upstream.

CC: @jrtc27 @resistor 

https://github.com/llvm/llvm-project/pull/165424