[InstCombine] Fold X sdiv (-1 << C) -> -(X u>> Y) iff X is non-negative

This is the one i'm seeing as missed optimization,
although there are likely other possibilities, as usual.

There are 4 variants of a general sdiv->udiv fold:
https://rise4fun.com/Alive/VS6

Name: v0
Pre: C0 >= 0 && C1 >= 0
%r = sdiv i8 C0, C1
  =>
%r = udiv i8 C0, C1

Name: v1
Pre: C0 <= 0 && C1 >= 0
%r = sdiv i8 C0, C1
  =>
%t0 = udiv i8 -C0, C1
%r = sub i8 0, %t0

Name: v2
Pre: C0 >= 0 && C1 <= 0
%r = sdiv i8 C0, C1
  =>
%t0 = udiv i8 C0, -C1
%r = sub i8 0, %t0

Name: v3
Pre: C0 <= 0 && C1 <= 0
%r = sdiv i8 C0, C1
  =>
%r = udiv i8 -C0, -C1


If we really don't like sdiv (more than udiv that is),
and are okay with increasing instruction count (2 new negations),
and we ensure that we don't undo the fold,
then we could just implement these..