Cleanup ConstantFoldCompareInstruction:

1. Make the arguments const like the other ConstantFold* functions.
2. Clean up evaluateFCmpRelation so it makes sense for floating point.
3. Implement the use of evaluateFCmpRelation to fold floating point CEs
4. Shorten a variable name so more things fit on one line.
5. Fix various comments.

llvm-svn: 32759