[CodeGen] MachineRegisterInfo::constrainRegAttrs - add explicit auto reference to prevent copy.

Fixes static analysis warning

Author: RKSimon

llvm/lib/CodeGen/MachineRegisterInfo.cpp

@@ -96,9 +96,9 @@ MachineRegisterInfo::constrainRegAttrs(Register Reg,
   if (RegTy.isValid() && ConstrainingRegTy.isValid() &&
       RegTy != ConstrainingRegTy)
     return false;
-  const auto ConstrainingRegCB = getRegClassOrRegBank(ConstrainingReg);
+  const auto &ConstrainingRegCB = getRegClassOrRegBank(ConstrainingReg);
   if (!ConstrainingRegCB.isNull()) {
-    const auto RegCB = getRegClassOrRegBank(Reg);
+    const auto &RegCB = getRegClassOrRegBank(Reg);
     if (RegCB.isNull())
       setRegClassOrRegBank(Reg, ConstrainingRegCB);
     else if (isa<const TargetRegisterClass *>(RegCB) !=