[InstCombine] use similar ops for related folds; NFCI

It's less efficient to produce 'ule' than 'ult' since we know we're going to
canonicalize to 'ult', but we shouldn't have duplicated code for these folds.

As a trade-off, this was a pretty terrible way to make a '2'. :)
       if (LHSC == SubOne(RHSC)) 
         AddC = ConstantExpr::getSub(AddOne(RHSC), LHSC);

The next steps are to share the code to fix PR32524 and add the missing 'and'
fold that was left out when PR14708 was fixed:
https://bugs.llvm.org/show_bug.cgi?id=14708

llvm-svn: 300222

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