Recommit "[InstCombine] Negator: -(X << C) --> X * (-1 << C)"

This reverts commit ac70b37a
which reverted commit 8aeb2fe1
because codegen tests got broken and i needed time to investigate.

This shows some regressions in tests, but they are all around GEP's,
so i'm not really sure how important those are.

https://rise4fun.com/Alive/1Gn