[CVP] Enhance SRem -> URem fold to work not just on non-negative operands

This is a continuation of 8d487668,
the logic is pretty much identical for SRem:

Name: pos pos
Pre: C0 >= 0 && C1 >= 0
%r = srem i8 C0, C1
  =>
%r = urem i8 C0, C1

Name: pos neg
Pre: C0 >= 0 && C1 <= 0
%r = srem i8 C0, C1
  =>
%r = urem i8 C0, -C1

Name: neg pos
Pre: C0 <= 0 && C1 >= 0
%r = srem i8 C0, C1
  =>
%t0 = urem i8 -C0, C1
%r = sub i8 0, %t0

Name: neg neg
Pre: C0 <= 0 && C1 <= 0
%r = srem i8 C0, C1
  =>
%t0 = urem i8 -C0, -C1
%r = sub i8 0, %t0

https://rise4fun.com/Alive/Vd6

Now, this new logic does not result in any new catches
as of vanilla llvm test-suite + RawSpeed.
but it should be virtually compile-time free,
and it may be important to be consistent in their handling,
because if we had a pair of sdiv-srem, and only converted one of them,
-divrempairs will no longer see them as a pair,
and thus not "merge" them.