Bitcode: Don't optimize constants when preserving use-list order

`ValueEnumerator::OptimizeConstants()` creates forward references within
the constant pools, which makes predicting constants' use-list order
difficult.  For now, just disable the optimization.

This can be re-enabled in the future in one of two ways:

  - Enable a limited version of this optimization that doesn't create
    forward references.  One idea is to categorize constants by their
    "height" and make that the top-level sort.

  - Enable it entirely.  This requires predicting how may times each
    constant will be recreated as its operands' and operands' operands'
    (etc.) forward references get resolved.

This is part of PR5680.