[InstCombine] fold mul with decremented "shl -1" factor

This is a sibling to:
6064e92b
...but we canonicalize the shl+add to shl+xor,
so the pattern is different than I expected:
https://alive2.llvm.org/ce/z/8CX16e

I have not found any patterns that are safe
to propagate no-wrap, so that is not included
here.