[InstCombine] Negator: cache negation results (PR46362)

It is possible that we can try to negate the same value multiple times.
For example, PHI nodes may happen to have multiple incoming values
(all of which must be the same value) for the same incoming basic block.
It may happen that we try to negate such a PHI node, and succeed,
and that might result in having now-different incoming values..

To avoid that, and in general to reduce the amount of duplicated
work we might be doing, let's introduce a cache where
we'll track results of negating each value.

The added test was previously failing -verify after -instcombine.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46362