This is the second attempt to move the stdu instruction in the prologue and epilogue.
The first attempt was D41737 but that contained a bug that was exposed by "make test" in libvpx.
In order to fix that issue the transformation is turned off for functions that require frame index scavenging. In order to determine that information the requiresFrameIndexScavenging had to be implemented for PowerPC.
callee saved instructions sounds odd. Perhaps CSR save/restore code?