[InstCombine] fold icmp of offset value with constant

There must be a better way to describe this pattern in words?
(X + C2) >u C --> X <s -C2 (if C == C2 + SMAX)

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

  define i1 @src(i8 %a, i8 %c1) {
    %t = add i8 %a, %c1
    %c2 = add i8 %c1, 127 ; SMAX
    %ov = icmp ugt i8 %t, %c2
    ret i1 %ov
  }

  define i1 @tgt(i8 %a, i8 %c1) {
    %neg_c1 = sub i8 0, %c1
    %ov = icmp slt i8 %a, %neg_c1
    ret i1 %ov
  }

The pattern was noticed as a by-product of D104932.