[PowerPC] Don't consider fusion in PPC64 address-formation peephole

The logic in this function assumes that the P8 supports fusion of addis/addi,
but it does not. As a result, there is no advantage to restricting our peephole
application, merging addi instructions into dependent memory accesses, even
when the addi has multiple users, regardless of whether or not we're optimizing
for size.

We might need something like this again for the P9; I suspect we'll revisit
this code when we work on P9 tuning.

llvm-svn: 280440