[ARM] Negative constants mishandled in ARM CGP

During type promotion, sometimes we convert negative an add with a
negative constant into a sub with a positive constant. The loop that
performs this transformation has two issues:
- it iterates over a set, causing non-determinism.
- it breaks, instead of continuing, when it finds the first
  non-negative operand.

Differential Revision: https://reviews.llvm.org/D58452

llvm-svn: 354557