[llvm] [Attributor] Use known non-flat AS before `getAssumedAddrSpace` (PR #143221)

[Matt Arsenault via llvm-commits]

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@@ -12592,19 +12592,38 @@ struct AAAddressSpaceImpl : public AAAddressSpace {
 
   ChangeStatus updateImpl(Attributor &A) override {
     uint32_t OldAddressSpace = AssumedAddressSpace;
+    unsigned FlatAS = A.getInfoCache().getFlatAddressSpace().value();
 
     auto CheckAddressSpace = [&](Value &Obj) {
-      if (isa<UndefValue>(&Obj))
+      // Ignore undef and poison.
+      if (isa<UndefValue>(&Obj) || isa<PoisonValue>(&Obj))
         return true;
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arsenm wrote:

`isa<UndefValue>` already covers undef and poison. However, I think in this context it's unsafe to handle undef. You shouldn't touch this in this PR 

https://github.com/llvm/llvm-project/pull/143221