Make FP tests requiring two compares work on PPC (PR 642).

This is Chris' patch from the PR, modified to realize that
SETUGT/SETULT occur legitimately with integers, plus
two fixes in LegalizeDAG to pass a valid result type into
LegalizeSetCC.  The argument of TLI.getSetCCResultType is
ignored on PPC, but I think I'm following usage elsewhere.

llvm-svn: 58871