swift234

Use dominance to remove redundant conditional branching.

This updates the CFG optimization pass to use the dominator tree to
eliminate redundant switch_enum instructions. It can be enhanced at a
later time to handle other conditional branching instructions.

Because jump threading can expose opportunitites for this, we run a pass
of CFG simplifications prior to running this, and since this exposes
more opportunities for those CFG simplifications, we run them again
after. Despite this, there is no measurable compile-time impact building
the stdlib with a release build of the compiler.

This in concert with the two commits that precede it reduce the runtime
of the string sort benchmark by 9%, RC4 by around 2-3%, and now the
generated SIL for optionals look slightly more sane.

Swift SVN r16510