[llvm] [VPlan] Remove unused first mask op from VPBlendRecipe. (PR #87770)

[via llvm-commits]

================
@@ -8245,6 +8245,8 @@ VPBlendRecipe *VPRecipeBuilder::tryToBlend(PHINode *Phi,
              "Distinct incoming values with one having a full mask");
       break;
     }
+    if (In == 0)
+      continue;
----------------
ayalz wrote:

nit: may be better to early-continue earlier - before creating the edge mask, which for first operand is needed only to assert, and for other operands can simplify the assert. Can also simplify simplifyRecipe() to look for single-operand Blends, by doing, e.g.,
```
  OperandsWithMask.push_back(Operands[0]);
  if (all_equal(Operands)) /* This case is optimized away later (can be simplified). */
    return new VPBlendRecipe(Phi, OperandsWithMask);

  /* First operand must have a non-full mask, which is ignored. */
  assert(createEdgeMask(Phi->getIncomingBlock(0), Phi->getParent()) &&
             "Distinct incoming values with one having a full mask");

  for (unsigned In = 1; In < NumIncoming; In++) {
    OperandsWithMask.push_back(Operands[In]);
    VPValue *EdgeMask =
        createEdgeMask(Phi->getIncomingBlock(In), Phi->getParent());
    assert(EdgeMask && "Both null and non-null edge masks found");
    OperandsWithMask.push_back(EdgeMask);
  }
  return new VPBlendRecipe(Phi, OperandsWithMask);
```

https://github.com/llvm/llvm-project/pull/87770