swift234

[constant-propagation] Refactor constant propagation slightly to disable diagnostics so we can use it in the performance passes to help with branch simplification.

This commit also enables constant propagation in the performance
pipeline.

Since we are close to WWDC, this commit purposefully minimally touches
the pass (despite my hands wanted to refactor it so bad) just enough so
that we get the desired result with minimal in tree turmoil.

rdar://16604715

Swift SVN r16388