[DSE] Enable the initializes improvement in DSE (#124058)

(Retry) enable the initializes improvement in DSE.

Initially enabled in llvm/llvm-project#119116.

Fix the aliasing issue through global variables in
llvm/llvm-project#120044.

The compile-time comparison of this enabling (no meaningful diff):
https://llvm-compile-time-tracker.com/compare.php?from=2f9f12d0ffaa36a5a23f26dda259f01364c318f1&to=33dc817b81f7898c87b052d1ddfd3d6e6f5b5dbd&stat=instructions%3Au

Author: haopliu

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -164,9 +164,9 @@ static cl::opt<bool>
     OptimizeMemorySSA("dse-optimize-memoryssa", cl::init(true), cl::Hidden,
                       cl::desc("Allow DSE to optimize memory accesses."));
 
-// TODO: turn on and remove this flag.
+// TODO: remove this flag.
 static cl::opt<bool> EnableInitializesImprovement(
-    "enable-dse-initializes-attr-improvement", cl::init(false), cl::Hidden,
+    "enable-dse-initializes-attr-improvement", cl::init(true), cl::Hidden,
     cl::desc("Enable the initializes attr improvement in DSE"));
 
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