[PowerPC] Add handling for conversions to fast-isel.

Yet another chunk of fast-isel code.  This one handles various
conversions involving floating-point.  (It also includes some
miscellaneous handling throughout the back end for LWA_32 and LWAX_32
that should have been part of the load-store patch.)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189677 91177308-0d34-0410-b5e6-96231b3b80d8