Cleanup ConstantFoldCompareInstruction:

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32759 91177308-0d34-0410-b5e6-96231b3b80d8