PPCInstrInfo::optimizeCompareInstr should not optimize FP compares

The floating-point record forms on PPC don't set the condition register bits
based on a comparison with zero (like the integer record forms do), but rather
based on the exception status bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181423 91177308-0d34-0410-b5e6-96231b3b80d8