AMDGPU: Stop trying to constrain register class of post-RA-pseudos (#161792)

This is trying to constrain the register class of a physical register,
which makes no sense.

Author: arsenm

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

@@ -2112,8 +2112,6 @@ bool SIInstrInfo::expandPostRAPseudo(MachineInstr &MI) const {
 
   case AMDGPU::SI_RESTORE_S32_FROM_VGPR:
     MI.setDesc(get(AMDGPU::V_READLANE_B32));
-    MI.getMF()->getRegInfo().constrainRegClass(MI.getOperand(0).getReg(),
-                                               &AMDGPU::SReg_32_XM0RegClass);
     break;
   case AMDGPU::AV_MOV_B32_IMM_PSEUDO: {
     Register Dst = MI.getOperand(0).getReg();