[InstCombine] canonicalize 'not' ops before logical shifts

This reverses the existing transform that would uniformly canonicalize any 'xor' after any shift. In the case of logical shifts, that turns a 'not' into an arbitrary 'xor' with constant, and that's probably not as good for analysis, SCEV, or codegen.

The SCEV motivating case is discussed in:
http://bugs.llvm.org/PR47136

There's an analysis motivating case at:
http://bugs.llvm.org/PR38781

I did draft a patch that would do the same for 'ashr' but that's questionable because it's just swapping the position of a 'not' and uncovers at least 2 missing folds that we would probably need to deal with as preliminary steps.

Alive proofs:
https://rise4fun.com/Alive/BBV

  Name: shift right of 'not'
  Pre: C2 == (-1 u>> C1)
  %a = lshr i8 %x, C1
  %r = xor i8 %a, C2
  =>
  %n = xor i8 %x, -1
  %r = lshr i8 %n, C1

  Name: shift left of 'not'
  Pre: C2 == (-1 << C1)
  %a = shl i8 %x, C1
  %r = xor i8 %a, C2
  =>
  %n = xor i8 %x, -1
  %r = shl i8 %n, C1

  Name: ashr of 'not'
  %a = ashr i8 %x, C1
  %r = xor i8 %a, -1
  =>
  %n = xor i8 %x, -1
  %r = ashr i8 %n, C1

Differential Revision: https://reviews.llvm.org/D86243