[ForwardOpTree] Introduce the -polly-optree pass.

This pass 'forwards' operand trees into statements that use them in
order to avoid scalar dependencies.

This minimal implementation handles only the case of speculatable
instructions. We will successively add support for:
- Hoisted loads
- Read-only values
- Synthesizable values
- Loads
- PHIs
- Forwarding only parts of the tree

Differential Revision: https://reviews.llvm.org/D35754

llvm-svn: 308825