[InstCombine] Fold icmps comparing uadd_sat with a constant

This patch is a continuation of D154206. It introduces a fold for the
operation "uadd_sat(X, C) pred C2" where "C" and "C2" are constants. The
fold is:

uadd_sat(X, C) pred C2
=> (X >= ~C) || ((X + C) pred C2) -> when (UINT_MAX pred C2) is true
=> (X < ~C)  && ((X + C) pred C2) -> when (UINT_MAX pred C2) is false

This patch also generalizes the fold to work with any saturating
intrinsic as long as the saturating value is known.

Proofs: https://alive2.llvm.org/ce/z/wWeirP

Differential Revision: https://reviews.llvm.org/D154565