[llvm] [LV] Add initial support for partial alias masking (PR #177599)

[Benjamin Maxwell via llvm-commits]

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@@ -46,11 +46,20 @@ VPValue *vputils::getOrCreateVPValueForSCEVExpr(VPlan &Plan, const SCEV *Expr) {
   if (U && !isa<Instruction>(U->getValue()))
     return Plan.getOrAddLiveIn(U->getValue());
   auto *Expanded = new VPExpandSCEVRecipe(Expr);
-  Plan.getEntry()->appendRecipe(Expanded);
+  VPBasicBlock *EntryVPBB = Plan.getEntry();
+  Plan.getEntry()->insert(Expanded, EntryVPBB->getFirstNonPhi());
   return Expanded;
 }
 
 bool vputils::isHeaderMask(const VPValue *V, const VPlan &Plan) {
+  if (V == &Plan.getAliasMask())
+    return true;
+
+  VPValue *Mask;
+  if (match(V,
+            m_c_BinaryAnd(m_VPValue(Mask), m_Specific(&Plan.getAliasMask()))))
+    V = Mask;
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MacDue wrote:

IIRC, this is needed to allow some plan verification and mask finding to work correctly. This function already heavily assumes what the IR looks like, I don't believe any more `ands` will be needed for this feature. 

https://github.com/llvm/llvm-project/pull/177599