swift234

[sil-combine] Initial SILCombine implementation.

This pass is a port of InstCombine from LLVM to SIL. Thus if you are familiar
with the code from InstCombine you will feel right at home.

Keep in mind that in order to help with review, this is just a skeleton with no
optimizations in it besides a simple DCE based off of isInstructionTriviallyDead
(which this patch exposes in Local.h like LLVM does) to ensure that trivial
testing of the pass can be accomplished since otherwise it would do nothing
implying that no tests could be written at all.

I additionally modified one test which no longer passed due to SILCombine
removing 1x unused metatype instruction from the standard library.

Swift SVN r9404