[ConstraintElimination] Make decompose() infallible

decompose() currently returns a mix of {} and 0 + 1*V on failure.
This changes it to always return the 0 + 1*V form, thus making
decompose() infallible.

This makes the code marginally more powerful, e.g. we now fold
sub_decomp_i80 by treating the constant as a symbolic value.

Differential Revision: https://reviews.llvm.org/D137847