[InstCombine] Move negation handling into freelyNegateValue()

Followup to D72978. This moves existing negation handling in
InstCombine into freelyNegateValue(), which make it composable.
In particular, root negations of div/zext/sext/ashr/lshr/sub can
now always be performed through a shl/trunc as well.

Differential Revision: https://reviews.llvm.org/D73288