[PowerPC] Fast-isel cleanup patch.

Here are a few miscellaneous things to tidy up the PPC64 fast-isel
implementation.  I corrected a couple of commentary lapses, and added
documentation of future opportunities.  I also implemented
TargetMaterializeAlloca, which I somehow forgot when I split up the
original huge patch.

Finally, I decided to delete SelectCmp.  I hadn't previously hooked it
in to TargetSelectInstruction(), and when I did I realized it wasn't
serving any useful purpose.  This is only useful for compares that
don't feed a branch in the same block, and to handle that we would
have to have logic to interpret i1 as a condition register.  This
could probably be done, but would require Unseemly Hackery, and
honestly does not seem worth the hassle.

This ends the current patch series.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189715 91177308-0d34-0410-b5e6-96231b3b80d8