AMDGPU: Sign extend constants when splitting them

[arsenm]

Summary

This will confuse later passes which try to look at the immediate value and don't truncate first.

[• tstellarAMD]

LGTM.

[arsenm]

r280974

lib/Target/AMDGPU/SIInstrInfo.cpp

 MachineOperand SIInstrInfo::buildExtractSubRegOrImm(
   MachineBasicBlock::iterator MII,
   MachineRegisterInfo &MRI,
   MachineOperand &Op,
   const TargetRegisterClass *SuperRC,
   unsigned SubIdx,
   const TargetRegisterClass *SubRC) const {
   if (Op.isImm()) {
-    // XXX - Is there a better way to do this?
     if (SubIdx == AMDGPU::sub0)
-      return MachineOperand::CreateImm(Op.getImm() & 0xFFFFFFFF);
+      return MachineOperand::CreateImm(static_cast<int32_t>(Op.getImm()));
     if (SubIdx == AMDGPU::sub1)
-      return MachineOperand::CreateImm(Op.getImm() >> 32);
+      return MachineOperand::CreateImm(static_cast<int32_t>(Op.getImm() >> 32));
 
     llvm_unreachable("Unhandled register index for immediate");
   }
 
   unsigned SubReg = buildExtractSubReg(MII, MRI, Op, SuperRC,
                                        SubIdx, SubRC);
   return MachineOperand::CreateReg(SubReg, false);
 }