[InstSimplify] try to eliminate icmp Pred (add nsw X, C1), C2

I was surprised to see that we're missing icmp folds based on 'add nsw' in InstCombine, 
but we should handle the InstSimplify cases first because that could make the InstCombine
code simpler.

Here are Alive-based proofs for the logic:

Name: add_neg_constant
Pre: C1 < 0 && (C2 > ((1<<(width(C1)-1)) + C1))
%a = add nsw i7 %x, C1
%b = icmp sgt %a, C2
  =>
%b = false

Name: add_pos_constant
Pre: C1 > 0 && (C2 < ((1<<(width(C1)-1)) + C1 - 1))
%a = add nsw i6 %x, C1
%b = icmp slt %a, C2
  =>
%b = false

Name: nuw
Pre: C1 u>= C2
%a = add nuw i11 %x, C1
%b = icmp ult %a, C2
  =>
%b = false

Differential Revision: https://reviews.llvm.org/D29053

llvm-svn: 292952