[MachineCopyPropagation] Eliminate spillage copies that might be caused by eviction chain

Remove spill-reload like copy chains. For example
```
r0 = COPY r1
r1 = COPY r2
r2 = COPY r3
r3 = COPY r4
<def-use r4>
r4 = COPY r3
r3 = COPY r2
r2 = COPY r1
r1 = COPY r0
```
will be folded into
```
r0 = COPY r1
r1 = COPY r4
<def-use r4>
r4 = COPY r1
r1 = COPY r0
```

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D122118

Author: Kai Luo

llvm/include/llvm/CodeGen/TargetSubtargetInfo.h

@@ -318,6 +318,11 @@ class TargetSubtargetInfo : public MCSubtargetInfo {
   classifyGlobalFunctionReference(const GlobalValue *GV) const {
     return 0;
   }
+
+  /// Enable spillage copy elimination in MachineCopyPropagation pass. This
+  /// helps removing redundant copies generated by register allocator when
+  /// handling complex eviction chains.
+  virtual bool enableSpillageCopyElimination() const { return false; }
 };
 
 } // end namespace llvm