[PostRASink] Add target hook shouldPostRASink (#167182)

Author: jgu222

llvm/include/llvm/CodeGen/TargetInstrInfo.h

@@ -436,7 +436,10 @@ class LLVM_ABI TargetInstrInfo : public MCInstrInfo {
   /// MachineSink determines on its own whether the instruction is safe to sink;
   /// this gives the target a hook to override the default behavior with regards
   /// to which instructions should be sunk.
+  ///
+  /// shouldPostRASink() is used by PostRAMachineSink.
   virtual bool shouldSink(const MachineInstr &MI) const { return true; }
+  virtual bool shouldPostRASink(const MachineInstr &MI) const { return true; }
 
   /// Return false if the instruction should not be hoisted by MachineLICM.
   ///

llvm/lib/CodeGen/MachineSink.cpp

@@ -2287,6 +2287,10 @@ bool PostRAMachineSinkingImpl::tryToSinkCopy(MachineBasicBlock &CurBB,
       continue;
     }
 
+    // Don't postRASink instructions that the target prefers not to sink.
+    if (!TII->shouldPostRASink(MI))
+      continue;
+
     if (MI.isDebugOrPseudoInstr())
       continue;