[InstCombine] Do not request non-splat vector support in code reviews (NFC) (#90709)

The InstCombine contributor guide already says:

> Handle non-splat vector constants if doing so is free, but do
> not add handling for them if it adds any additional complexity
> to the code.

This change strengthens this guideline to explicitly discourage
asking (new) contributors to implement non-splat support during code
reviews. Doing so will almost certainly increase the number of
necessary review iterations, or result in outright contradictory review
feedback, as different people are willing to accept a different degree
of complexity for non-splat vector support.