[ConstraintElim] Replace NUWSub decomp with recursive decomp of ops.

The current patterns for NUWSub decompositions do not handle negative
constants correctly at the moment (causing #76713).

Replace the incorrect pattern by more general code that recursively
decomposes the operands and then combines the results. This is already
done in most other places that handle operators like add/mul.

This means we fall back to the general constant handling code (fixes the
mis-compile) while also being able to support reasoning about
decomposable expressions in the SUB operands.

Fixes https://github.com/llvm/llvm-project/issues/76713.