[AVR] Revert the functional part of r291083

As Senthil points out, this is unnecessary as we already have these
registers in AddlRegNames.

llvm-svn: 291090
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index 8f38ab6..b77233c 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -8446,8 +8446,7 @@
       "r0",   "r1",   "r2",   "r3",   "r4",   "r5",   "r6",   "r7",
       "r8",   "r9",   "r10",  "r11",  "r12",  "r13",  "r14",  "r15",
       "r16",  "r17",  "r18",  "r19",  "r20",  "r21",  "r22",  "r23",
-      "r24",  "r25",  "r26",  "r27",  "r28",  "r29",  "r30",  "r31",
-      "X",    "Y",    "Z",    "SP"
+      "r24",  "r25",  "X",    "Y",    "Z",    "SP"
     };
     return llvm::makeArrayRef(GCCRegNames);
   }