Fix eabi calling convention when a 64 bit value shadows r3.

Without this what was happening was:

* R3 is not marked as "used"
* ARM backend thinks it has to save it to the stack because of vaarg
* Offset computation correctly ignores it
* Offsets are wrong

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110446 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/CallingConvLower.h

@@ -275,6 +275,12 @@ public:
     return Result;
   }
 
+  /// Version of AllocateStack with extra register to be shadowed.
+  unsigned AllocateStack(unsigned Size, unsigned Align, unsigned ShadowReg) {
+    MarkAllocated(ShadowReg);
+    return AllocateStack(Size, Align);
+  }
+
   // HandleByVal - Allocate a stack slot large enough to pass an argument by
   // value. The size and alignment information of the argument is encoded in its
   // parameter attribute.