[AMDGPU] Avoid RegScavenger::forward in copyPhysReg/indirectCopyToAGPR

[foad]

Summary

RegScavenger::backward is preferred because it does not rely on accurate
kill flags.

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

for (auto Def = MI, E = MBB.begin(); Def != E; ) {
         Builder.addReg(ImpUseSuperReg,
                       getKillRegState(KillSrc) | RegState::Implicit);
       }
 
       return;
     }
   }
 
-  RS.enterBasicBlock(MBB);
-  RS.forward(MI);
+  RS.enterBasicBlockEnd(MBB);
+  RS.backward(MI);
 
   // Ideally we want to have three registers for a long reg_sequence copy
   // to hide 2 waitstates between v_mov_b32 and accvgpr_write.
   unsigned MaxVGPRs = RI.getRegPressureLimit(&AMDGPU::VGPR_32RegClass,
                                              *MBB.getParent());
 
   // Registers in the sequence are allocated contiguously so we can just
   // use register number to pick one of three round-robin temps.