[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. 

llvm-svn: 278828