[InstCombine] generalize fold for mask-with-signbit-splat, part 2

This removes an over-specified fold. The more general transform
was added with:
727e642e

There's a difference on an existing test that shows a potentially
unnecessary use limit on an icmp fold.

That fold is in InstCombinerImpl::foldICmpSubConstant(), and IIRC
there was some back-and-forth on it and similar folds because they
could cause analysis/passes (SCEV, LSR?) to miss optimizations.

Differential Revision: https://reviews.llvm.org/D111410