[PowerPC] Add getCRSaveOffset to improve readability. [NFC]

In preperation for AIX support in FrameLowering: replace a number of literal
'8' that represent the stack offset of the condition register save area with
a member in PPCFrameLowering.

Patch by Chris Bowler.

llvm-svn: 367111
diff --git a/llvm/lib/Target/PowerPC/PPCFrameLowering.h b/llvm/lib/Target/PowerPC/PPCFrameLowering.h
index d116e9f..7783a05 100644
--- a/llvm/lib/Target/PowerPC/PPCFrameLowering.h
+++ b/llvm/lib/Target/PowerPC/PPCFrameLowering.h
@@ -26,6 +26,7 @@
   const unsigned FramePointerSaveOffset;
   const unsigned LinkageSize;
   const unsigned BasePointerSaveOffset;
+  const unsigned CRSaveOffset;
 
   /**
    * Find register[s] that can be used in function prologue and epilogue
@@ -152,6 +153,10 @@
   /// base pointer.
   unsigned getBasePointerSaveOffset() const { return BasePointerSaveOffset; }
 
+  /// getCRSaveOffset - Return the previous frame offset to save the
+  /// CR register.
+  unsigned getCRSaveOffset() const { return CRSaveOffset; }
+
   /// getLinkageSize - Return the size of the PowerPC ABI linkage area.
   ///
   unsigned getLinkageSize() const { return LinkageSize; }