[llvm] [SLP] NFC. Use InstructionsState::valid if users just want to know whether VL has same opcode. (PR #120217)

[Alexey Bataev via llvm-commits]

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@@ -821,7 +821,8 @@ class InstructionsState {
 
   /// The main/alternate opcodes for the list of instructions.
   unsigned getOpcode() const {
-    return MainOp ? MainOp->getOpcode() : 0;
+    assert(MainOp && "InstructionsState is invalid.");
+    return MainOp->getOpcode();
   }
 
   unsigned getAltOpcode() const {
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alexey-bataev wrote:

And in isAltShuffle too

https://github.com/llvm/llvm-project/pull/120217