[InstCombine] refactor fabs+fcmp fold; NFC

Also, remove/replace/minimize/enhance the tests for this fold.
The code drops FMF, so it needs more tests and at least 1 fix.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345734 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed