[PowerPC] Move the stack pointer update instruction later in the prologue and earlier in the epilogue.
Move the stdu instruction in the prologue and epilogue.
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
Differential Revision: https://reviews.llvm.org/D42590