[GlobalISel] Ensure MachineIRBuilder::getDebugLoc() returns a const reference. NFCI.

Avoids a lot of unnecessary tracking increments/decrements of the underlying TrackingMDNodeRef.

llvm/include/llvm/CodeGen/GlobalISel/MachineIRBuilder.h

@@ -363,7 +363,7 @@ public:
   void setDebugLoc(const DebugLoc &DL) { this->State.DL = DL; }
 
   /// Get the current instruction's debug location.
-  DebugLoc getDebugLoc() { return State.DL; }
+  const DebugLoc &getDebugLoc() { return State.DL; }
 
   /// Build and insert <empty> = \p Opcode <empty>.
   /// The insertion point is the one set by the last call of either