fix[compiler playground]: Set source as the pre-change state in HIR d…

…iff (#29957)

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## Summary

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Explain the **motivation** for making this change. What existing problem
does the pull request solve?
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I have fixed an issue where the display of the HIR diff in the React
Compiler Playground was incorrect. The HIR diff is supposed to show the
pre-change state as the source, but currently, it is showing
EnvironmentConfig as the pre-change state. This PR corrects this by
setting the pre-change state to source instead of EnvironmentConfig.

## How did you test this change?

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before:


![image](https://github.com/facebook/react/assets/37236438/c98aee7a-b569-430e-8698-0f7eff6c21e2)

after:


![image](https://github.com/facebook/react/assets/37236438/6113297c-80f9-4e71-a0d1-df8cc87507db)

compiler/apps/playground/components/Editor/Output.tsx

@@ -104,7 +104,7 @@ async function tabify(source: string, compilerOutput: CompilerOutput) {
       passName,
       <TextTabContent
         output={text}
-        diff={lastPassOutput ?? null}
+        diff={passName !== "HIR" ? lastPassOutput : null}
         showInfoPanel={true}
       ></TextTabContent>
     );
@@ -195,7 +195,7 @@ function Output({ store, compilerOutput }: Props) {
       if (result.kind === "hir" || result.kind === "reactive") {
         currResult += `function ${result.fnName}\n\n${result.value}`;
       }
-      if (currResult !== lastResult) {
+      if (passName !== "HIR" && currResult !== lastResult) {
         changedPasses.add(passName);
       }
       lastResult = currResult;