[PowerPC] Add integer truncation support to fast-isel.

This is the last substantive patch I'm planning for fast-isel in the
near future, adding fast selection of integer truncates.  There are
certainly more things that can be improved (many of which are called
out in FIXMEs), but for now we are catching most of the important
cases.

I'll document some of the remaining work in a cleanup patch shortly.