[InstCombine] optimize unsigned icmp of increment

Allows LLVM to optimize sequences like the following:

%add = add nuw i32 %x, 1
%cmp = icmp ugt i32 %add, %y

Into:

%cmp = icmp uge i32 %x, %y

Previously, only signed comparisons were being handled.

Decrements could also be handled, but 'sub nuw %x, 1' is currently canonicalized to
'add %x, -1' in InstCombineAddSub, losing the nuw flag. Removing that canonicalization
seems like it might have far-reaching ramifications so I kept this simple for now.

Patch by Matti Niemenmaa!

Differential Revision: https://reviews.llvm.org/D24700

llvm-svn: 291975