[llvm] [IR][TLI] Cache getLibFunc() result on Function (NFC) (PR #72867)

[Arthur Eubanks via llvm-commits]

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@@ -913,6 +913,7 @@ Intrinsic::ID Function::lookupIntrinsicID(StringRef Name) {
 }
 
 void Function::recalculateIntrinsicID() {
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aeubanks wrote:

should make the method name more generic, like `updateAfterNameChange`

https://github.com/llvm/llvm-project/pull/72867