Re-apply "[AArch64] Enable "sink-and-fold" in MachineSink by default (#67432)"

This re-applies commit ace20e2, which was reverted in eff4ef2.

The issues were fixed in:

  * b30765c [AArch64] Fix an incorrect handling of debug values in
    MachineSink (#68107)

  * b454b04 [AArch64] Fix a compiler crash in MachineSink (#67705)

Author: momchil-velikov

llvm/lib/Target/AArch64/AArch64TargetMachine.cpp

@@ -200,7 +200,7 @@ static cl::opt<bool> EnableGISelLoadStoreOptPostLegal(
 static cl::opt<bool>
     EnableSinkFold("aarch64-enable-sink-fold",
                    cl::desc("Enable sinking and folding of instruction copies"),
-                   cl::init(false), cl::Hidden);
+                   cl::init(true), cl::Hidden);
 
 extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeAArch64Target() {
   // Register the target.

llvm/test/CodeGen/AArch64/sink-and-fold.ll

@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc -aarch64-enable-sink-fold=true < %s | FileCheck %s
+; RUN: llc < %s | FileCheck %s
 target triple = "aarch64-linux"
 
 declare i32 @use(...)