[llvm] [SPIRV] Enable DCE in instruction selection and update tests (PR #168428)

[Nathan Gauër via llvm-commits]

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@@ -94,6 +94,9 @@ class SPIRVInstructionSelector : public InstructionSelector {
 
 private:
   void resetVRegsType(MachineFunction &MF);
+  // New helper function for dead instruction removal
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Keenuts wrote:

nit: remove? comment doesn't add much value no? Or maybe explain what those 2 functions are assuming/doing?

https://github.com/llvm/llvm-project/pull/168428