[InstCombine] allow Negator to fold multi-use select with constant arms

The motivating test is reduced from:
https://llvm.org/PR52261

Note that the more general problem of folding any binop into a multi-use
select of constants is still there. We need to ease the restriction in
InstCombinerImpl::FoldOpIntoSelect() to catch those. But these examples
never reach that code because Negator exclusively handles negation
patterns within visitSub().

Differential Revision: https://reviews.llvm.org/D112657