[InstCombine] fix formatting for foldLogOpOfMaskedICmps and related bits; NFCI

1. Improve enum, function, and variable names.
2. Improve comments.
3. Fix variable capitalization.
4. Run clang-format.

As an existing code comment suggests, this should work with vector types / splat constants too,
so making this look right first will reduce the diffs needed for that change.

llvm-svn: 299365