swift234

[let-properties-opt] Re-factor and simplify the code. Improve correctness.

Based on the review of my previous commit, I did some re-factorings.
The code is now simpler and handles more cases. More tests were added.

The overall idea of this rewrite is that the pass basically tries to check if it can
see all possible writes (i.e. initializations) into a given let property. Only if it can be proven
that the pass sees all possible writes and all those initializations are producing
the same constant, statically known value, the pass propagates this constant value
into uses of a property.

SR-1026 and rdar://25303106