[InstCombine] convert mul by negative-pow2 to negate and shift

This is an unusual canonicalization because we create an extra instruction,
but it's likely better for analysis and codegen (similar reasoning as D133399).

InstCombine::Negator may create this kind of multiply from negate and shift,
but this should not conflict because of the narrow negation.

I don't know how to create a fully general proof for this kind of transform in
Alive2, but here's an example with bitwidths similar to one of the regression
tests:
https://alive2.llvm.org/ce/z/J3jTjR

Differential Revision: https://reviews.llvm.org/D133667