swift234

[sink] Add an optimization pass for sinking code.

Some instructions are side-effect free at the SIL-level, but may read or write
memory at the LLVM level, so we can't rely on the llvm sinking pass to sink
them. This pass sinks side-effect free instructions, such as 'upcast' and
'class_method' to a basic block that dominates all of their users.

Swift SVN r31868