[InstCombine] fold icmp ult of offset value with constant

This is one sibling of the fold added with c7b658ae .

(X + C2) <u C --> X >s ~C2 (if C == C2 + SMIN)
I'm still not sure how to describe it best, but we're
translating 2 constants from an unsigned range comparison
to signed because that eliminates the offset (add) op.

This could be extended to handle the more general (non-constant)
pattern too:
https://alive2.llvm.org/ce/z/K-fMBf

  define i1 @src(i8 %a, i8 %c2) {
    %t = add i8 %a, %c2
    %c = add i8 %c2, 128 ; SMIN
    %ov = icmp ult i8 %t, %c
    ret i1 %ov
  }

  define i1 @tgt(i8 %a, i8 %c2) {
    %not_c2 = xor i8 %c2, -1
    %ov = icmp sgt i8 %a, %not_c2
    ret i1 %ov
  }