[InstCombine] add helper functions for foldICmpWithConstant; NFCI
Besides breaking up a 700 line function to improve readability,
this sinks the 'FIXME: ConstantInt' check into each helper. So
now we can independently break that restriction within any of the
helper functions.
As much as possible, the code was only {cut/paste/clang-format}'ed
to minimize risk (no functional changes intended), so several more
readability improvements are still possible.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278828
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