[GISel]: IRTranslator: Prefer a buidInstr form that allows CSE of cast instructions

[aditya_nandakumar]

Summary

Use the style of buildInstr that allows CSEing.

[arsenm]

LGTM

[aditya_nandakumar]

r358637

lib/CodeGen/GlobalISel/IRTranslator.cpp

bool IRTranslator::translateBitCast(const User &U,
   }
   return translateCast(TargetOpcode::G_BITCAST, U, MIRBuilder);
 }
 
 bool IRTranslator::translateCast(unsigned Opcode, const User &U,
                                  MachineIRBuilder &MIRBuilder) {
   unsigned Op = getOrCreateVReg(*U.getOperand(0));
   unsigned Res = getOrCreateVReg(U);
-  MIRBuilder.buildInstr(Opcode).addDef(Res).addUse(Op);
+  MIRBuilder.buildInstr(Opcode, {Res}, {Op});
   return true;
 }
 
 bool IRTranslator::translateGetElementPtr(const User &U,
                                           MachineIRBuilder &MIRBuilder) {
   // FIXME: support vector GEPs.
   if (U.getType()->isVectorTy())
     return false;