Revert "[AArch64] Enable Spillage Copy Elimination by default" (llvm#211572)

Reverts llvm#186093

Issue llvm#206839 shows that
EliminateSpillageCopies is not currently ready to be enabled by default
on AArch64. llvm#207169 was raised
to fix the issue, but concerns were raised around complexity of the
function and change. Until a suitable solution can be found,
EliminateSpillageCopy will be disabled on AArch64 by default.

For those who wish to still use the pass, `-enable-spill-copy-elim=true`
will ensure the pass is run.

(cherry picked from commit debd8a7)

Author: Stylie777

llvm/lib/Target/AArch64/AArch64Subtarget.h

@@ -160,7 +160,6 @@ class AArch64Subtarget final : public AArch64GenSubtargetInfo {
   bool enableMachineScheduler() const override { return true; }
   bool enablePostRAScheduler() const override { return usePostRAScheduler(); }
   bool enableSubRegLiveness() const override { return EnableSubregLiveness; }
-  bool enableSpillageCopyElimination() const override { return true; }
 
   bool enableMachinePipeliner() const override;
   bool useDFAforSMS() const override { return false; }