This should provide a small performance boost in functions that are able to do this.
I've kept this change rather conservative at the moment and functions with frame pointers or base pointers will not try to move the stack pointer update.
Performance numbers on Power 8 for SPEC 2006
429.mcf - 1.5 % improvement
462.libquantum - 1.0 % improvement
464.h264ref - 3.0 % improvement
471.omnetpp - 1.0 % improvement
453.povray - 1.5 % improvement
All other benchmarks changed less than 0.2 % which I consider as "no change".
I have found no degradations in SPEC2006.