[llvm] [SPIR-V] Guard against infinite loop in decorateUsesAsNonUniform (PR #208224)

[Arseniy Obolenskiy via llvm-commits]

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@@ -6317,10 +6318,14 @@ bool SPIRVInstructionSelector::selectResourceNonUniformIndex(
 
 void SPIRVInstructionSelector::decorateUsesAsNonUniform(
     Register &NonUniformReg) const {
-  llvm::SmallVector<Register> WorkList = {NonUniformReg};
+  llvm::SmallVector<std::pair<Register, MachineInstr *>> WorkList = {
+      {NonUniformReg, nullptr}};
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aobolensk wrote:

> There is no instruction defining NonUniformReg?

That seems there could be several instructions actually defining that. So, it seems it is better to observe them in a lazy way

https://github.com/llvm/llvm-project/pull/208224