[PowerPC] Use 16-byte alignment for modern cores for functions/loops

[PowerPC] Use 16-byte alignment for modern cores for functions/loops

Most modern PowerPC cores prefer that functions and loops start on
16-byte-aligned boundaries (*), so instruct block placement, etc. to make this
happen. The branch selector has also been adjusted so account for the extra
nops that might now be inserted before loop headers.

(*) Some cores actually prefer other alignments for small loops, but that will
    be addressed in a follow-up commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225115 91177308-0d34-0410-b5e6-96231b3b80d8