[ForwardOpTree] Use known array content analysis to forward load instructions.

This is an addition to the -polly-optree pass that reuses the array
content analysis from DeLICM to find array elements that contain the
same value as the value loaded when the target statement instance
is executed.

The analysis is now enabled by default.

The known content analysis could also be used to rematerialize any
llvm::Value that was written to some array element, but currently
only loads are forwarded.

Differential Revision: https://reviews.llvm.org/D36380

llvm-svn: 310279