Implements tree loop unroller using the infrastructure provided.

gcc/ChangeLog:

2018-02-12  Kugan Vivekanandarajah  <kuganv@linaro.org>

    * Makefile.in (OBJS): Add tree-ssa-loop-unroll.o.
    * common.opt (ftree-loop-unroll): New option.
    * passes.def: Add pass_tree_loop_uroll
    * timevar.def (TV_TREE_LOOP_UNROLL): Add.
    * tree-pass.h (make_pass_tree_loop_unroll): Declare.
    * tree-ssa-loop-unroll.c: New file.