[PowerPC] Don't mark the return-address slot as immutable

It is tempting to mark the fixed stack slot used to store the return address as
immutable when lowering @llvm.returnaddress(i32 0). Unfortunately, within the
function, it is not completely immutable: it is written during the function
prologue. When using post-RA instruction scheduling, the prologue instructions
are available for scheduling, and we're not free to interchange the order of a
particular store in the prologue with loads from that stack location.

Fixes PR21976.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224761 91177308-0d34-0410-b5e6-96231b3b80d8