[InstCombine] Fold icmps comparing usub_sat with a constant

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

usub_sat(X, C) pred C2
=> (X < C)  || ((X - C) pred C2) -> when (0 pred C2) is true
=> (X >= C) && ((X - C) pred C2) -> when (0 pred C2) is false

These expressions can generally be folded into a simpler expression. As
they can sometimes emit more than one instruction, they are limited to
cases where the "usub_sat" has only one user.

Fixes #58342.

Proofs: https://alive2.llvm.org/ce/z/ws_N2J

Differential Revision: https://reviews.llvm.org/D154206