Add a comment about overlapping PPC frame offsets

I don't think that it is otherwise clear how the overlapping offsets
are processed into distinct spill slots. Comment that this is done
in processFunctionBeforeFrameFinalized.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177094 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCFrameLowering.h

@@ -144,6 +144,9 @@ public:
       return 0;
     }
 
+    // Note that the offsets here overlap, but this is fixed up in
+    // processFunctionBeforeFrameFinalized.
+
     static const SpillSlot Offsets[] = {
       // Floating-point register save area offsets.
       {PPC::F31, -8},