[AVR] Fixed bug caused by optimizations

Whenever optimizations are turned on, compilation would fail.
This is was because the greedy register allocator was being used
regardless of optimization level. This caused an assertion error when
the LiveVariables pass is run.

Closes #15.

Author: Dylan McKay

lib/Target/AVR/AVRTargetMachine.cpp

@@ -57,7 +57,6 @@ class AVRPassConfig : public TargetPassConfig
   void addPreSched2() override;
   void addPreRegAlloc() override;
   void addPreEmitPass() override;
-  FunctionPass *createTargetRegisterAllocator(bool Optimized);
 };
 } // namespace
 
@@ -111,8 +110,3 @@ void AVRPassConfig::addPreEmitPass()
   addPass(createAVRBranchSelectionPass());
 }
 
-FunctionPass *AVRPassConfig::createTargetRegisterAllocator(bool Optimized)
-{
-  // Unconditionally use our custom greedy register allocator.
-  return createGreedyRegisterAllocator();
-}