[VPlan] Pick optimal seed value for VPBlend to enable mask elimination.

[paulwalker-arm]

Summary

The seed value for a VPBlend is not masked. By picking an incoming
value whose mask is only used by the VPBlend as the seed value, we
are able to dead code the value's mask.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

void VPBlendRecipe::execute(VPTransformState &State) {
   // We know that all PHIs in non-header blocks are converted into
   // selects, so we don't have to worry about the insertion order and we
   // can just use the builder.
   // At this point we generate the predication tree. There may be
   // duplications since this is a simple recursive scan, but future
   // optimizations will clean it up.
 
   unsigned NumIncoming = getNumIncomingValues();
+  unsigned StartInValIdx = 0;
+
+  // We might have single edge PHIs (blocks)
+  if (NumIncoming > 1) {
+    // Find an incoming value whose mask is only used by the blend. By seeding
+    // the blend with this value it's mask is not used and can be deadcoded.
+    for (unsigned In = 0; In < NumIncoming; ++In) {
+      if (getMask(In)->getNumUsers() == 1) {
+        StartInValIdx = In;
+        break;
+      }
+    }
+  }
 
   // Generate a sequence of selects of the form:
   // SELECT(Mask3, In3,
   //        SELECT(Mask2, In2,
   //               SELECT(Mask1, In1,
   //                      In0)))
   // Note that Mask0 is never used: lanes for which no path reaches this phi and
   // are essentially undef are taken from In0.
   VectorParts Entry(State.UF);
+
+  // Initialize with the incoming value whose mask can be esily deadcoded.
+  for (unsigned Part = 0; Part < State.UF; ++Part) {
+    Entry[Part] = State.get(getIncomingValue(StartInValIdx), Part);
+  }
+
   for (unsigned In = 0; In < NumIncoming; ++In) {
+    if (In == StartInValIdx)
+      continue;
+
     for (unsigned Part = 0; Part < State.UF; ++Part) {
-      // We might have single edge PHIs (blocks) - use an identity
-      // 'select' for the first PHI operand.
       Value *In0 = State.get(getIncomingValue(In), Part);
-      if (In == 0)
-        Entry[Part] = In0; // Initialize with the first incoming value.
-      else {
-        // Select between the current value and the previous incoming edge
-        // based on the incoming mask.
-        Value *Cond = State.get(getMask(In), Part);
-        Entry[Part] =
-            State.Builder.CreateSelect(Cond, In0, Entry[Part], "predphi");
-      }
-    }
-  }
   for (unsigned Part = 0; Part < State.UF; ++Part)
     State.set(this, Entry[Part], Part);
 }
 
 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
 void VPBlendRecipe::print(raw_ostream &O, const Twine &Indent,
                           VPSlotTracker &SlotTracker) const {
   O << Indent << "BLEND ";