[ForwardOpTree] Execute canForwardTree also in release builds.

Commit r309730 moved the call to canForwardTree into an assert(), even
though this function has side-effects if its DoIt parameter is true. To
avoid a warning in release builds, do an (void)Execution of its result
instead.

To avoid such confusion in the future, rename
canForwardTree() to forwardTree().

llvm-svn: 309753