[InstCombine] canonicalizeSaturatedAdd(): last fold is only valid for strict comparison (PR48390)

We could create uadd.sat under incorrect circumstances
if a select with -1 as the false value was canonicalized
by swapping the T/F values. Unlike the other transforms
in the same function, it is not invariant to equality.

Some alive proofs: https://alive2.llvm.org/ce/z/emmKKL

Based on original patch by David Green!

Fixes https://bugs.llvm.org/show_bug.cgi?id=48390

Differential Revision: https://reviews.llvm.org/D92717