[PowerPC] MachineSSA pass to reduce the number of CR-logical operations

The initial implementation of an MI SSA pass to reduce cr-logical operations.
Currently, the only operations handled by the pass are binary operations where
both CR-inputs come from the same block and the single use is a conditional
branch (also in the same block).

Committing this off by default to allow for a period of field testing. Will
enable it by default in a follow-up patch soon.

Differential Revision: https://reviews.llvm.org/D30431

llvm-svn: 320584