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

[Nathan Gauër via llvm-commits]

================
@@ -506,22 +509,195 @@ static bool isConstReg(MachineRegisterInfo *MRI, Register OpReg) {
   return false;
 }
 
+// TODO(168736): We should make this either a flag in tabelgen
+// or reduce our dependence on the global registery, so we can remove this
----------------
Keenuts wrote:

```suggestion
// or reduce our dependence on the global registry, so we can remove this
```

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