Add a FIXME about access to negative stack-pointer offsets on PPC32.

The current code will generate a prologue which starts with something like:
        mflr 0
        stw 31, -4(1)
        stw 0, 4(1)
        stwu 1, -16(1)

But under the PPC32 SVR4 ABI, access to negative offsets from R1 is not allowed.

This was pointed out by Peter Bergner.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157133 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCFrameLowering.cpp

@@ -330,6 +330,8 @@ void PPCFrameLowering::emitPrologue(MachineFunction &MF) const {
       BuildMI(MBB, MBBI, dl, TII.get(PPC::MFLR), PPC::R0);
 
     if (HasFP)
+      // FIXME: On PPC32 SVR4, FPOffset is negative and access to negative
+      // offsets of R1 is not allowed.
       BuildMI(MBB, MBBI, dl, TII.get(PPC::STW))
         .addReg(PPC::R31)
         .addImm(FPOffset)