[CVP] Soften SDiv into a UDiv as long as we know domains of both of the operands.

Yes, if operands are non-positive this comes at the extra cost
of two extra negations. But  a. division is already just
ridiculously costly, two more subtractions can't hurt much :)
and  b. we have better/more analyzes/folds for an unsigned division,
we could end up narrowing it's bitwidth, converting it to lshr, etc.

This is essentially a take two on 0fdcca07,
which didn't fix the potential regression i was seeing,
because ValueTracking's computeKnownBits() doesn't make use
of dominating conditions in it's analysis.
While i could teach it that, this seems like the more general fix.

This big hammer actually does catch said potential regression.

Over vanilla test-suite + RawSpeed + darktable
(10M IR instrs, 1M IR BB, 1M X86 ASM instrs), this fires/converts 5 more
(+2%) SDiv's, the total instruction count at the end of middle-end pipeline
is only +6, so out of +10 extra negations, ~half are folded away,
and asm instr count is only +1, so practically speaking all extra
negations are folded away and are therefore free.
Sadly, all these new UDiv's remained, none folded away.
But there are two less basic blocks.

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