[InstCombine] try to fold mul by neg-power-of-2 to shl

`(A * -2**C) + B --> B - (A << C)`

https://alive2.llvm.org/ce/z/A6BWkf

This inverts what Negator was doing before:
D134310 / 0f32a5de

Analysis and codegen are generally better without multiply,
so we should favor this form even if we trade add for sub
(because those are generally equivalent cost operations).