[Planning] Avoid batching compile job twice

When using incremental build, the FirstWaveComputer has already batched
all the compile jobs. `planStandardCompile()` can just reuse the batch
job information from incremental state to construct the jobs.

Author: cachemeifyoucan

Sources/SwiftDriver/Jobs/Planning.swift

@@ -90,16 +90,17 @@ extension Driver {
       incrementalCompilationState = nil
     }
 
-    return try (
-      // For compatibility with swiftpm, the driver produces batched jobs
-      // for every job, even when run in incremental mode, so that all jobs
-      // can be returned from `planBuild`.
-      // But in that case, don't emit lifecycle messages.
-      formBatchedJobs(jobsInPhases.allJobs,
-                      showJobLifecycle: showJobLifecycle && incrementalCompilationState == nil,
-                      jobCreatingPch: jobsInPhases.allJobs.first(where: {$0.kind == .generatePCH})),
-      incrementalCompilationState
-    )
+    let batchedJobs: [Job]
+    // If the jobs are batched during the incremental build, reuse the computation rather than computing the batches again.
+    if let incrementalState = incrementalCompilationState {
+      batchedJobs = incrementalState.mandatoryJobsInOrder + incrementalState.jobsAfterCompiles
+    } else {
+      batchedJobs = try formBatchedJobs(jobsInPhases.allJobs,
+                                        showJobLifecycle: showJobLifecycle,
+                                        jobCreatingPch: jobsInPhases.allJobs.first(where: {$0.kind == .generatePCH}))
+    }
+
+    return (batchedJobs, incrementalCompilationState)
   }
 
   /// If performing an explicit module build, compute an inter-module dependency graph.