[llvm] [VPlan] Add m_Deferred. NFC (PR #133736)

[Luke Lau via llvm-commits]

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@@ -66,6 +66,27 @@ struct specificval_ty {
 
 inline specificval_ty m_Specific(const VPValue *VPV) { return VPV; }
 
+/// Stores a reference to the VPValue *, not the VPValue * itself,
+/// thus can be used in commutative matchers.
+template <typename Class> struct deferredval_ty {
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lukel97 wrote:

The template is to allow matching both VPValue and const VPValue, which ties into your review comment below. If we don't think we'll ever need to match a const VPValue I can remove that overload and untemplate this function. 

https://github.com/llvm/llvm-project/pull/133736