swift234

[cse] Initial implementation of EarlyCSE Port.

This patch contains an initial implementation of CSE for SIL ported from LLVM's
earlycse pass. It follows the overall general structure using
ScopedHashTables/DominatorTrees to perform the computation.

Currently it only handles integer literals to ease with the initial review. Once
this gets in I will be spending some time extending the pass to handle other
instructions.

Even with just handling integers this reduces the size of the stdlib in SIL by
~300 lines.

Swift SVN r10794