[AVR] Fix register numbers for in getGCCAddlRegNames()

[dylanmckay]

Summary

These do not match up with the register numbers defined in LLVM's
AVRRegisterInfo.td

Adding Senthil as a reviewer as he originally implemented this.

[saaadhu]

If you've added X, Y, Z and SP to GCCRegNames, you don't need AddlRegNames array at all,

The reason I had them in AddlRegNames was to tell Clang that they alias regs in GCCRegNames. I followed X86TargetInfo's example - it has "ax" in GCCRegNames, and "al", "ah", "eax" and "rax" in AddlRegNames. I figured Clang could potentially use the fact that they alias when analyzing inline asm to detect unintended clobbers.

lib/Basic/Targets.cpp

public:
 
   ArrayRef<TargetInfo::GCCRegAlias> getGCCRegAliases() const override {
     return None;
   }
 
   ArrayRef<TargetInfo::AddlRegName> getGCCAddlRegNames() const override {
     static const TargetInfo::AddlRegName AddlRegNames[] = {
       { { "r26", "r27"}, 26 },
-      { { "r28", "r29"}, 27 },
-      { { "r30", "r31"}, 28 },
-      { { "SPL", "SPH"}, 29 },
+      { { "r28", "r29"}, 28 },
+      { { "r30", "r31"}, 30 },
+      { { "SPL", "SPH"}, 32 },
     };
     return llvm::makeArrayRef(AddlRegNames);
   }
 
   bool validateAsmConstraint(const char *&Name,
                              TargetInfo::ConstraintInfo &Info) const override {
     return false;
   }