Add a pass to back-propagate use information

This patch adds a pass that collects information that is common to
all uses of an SSA name X and back-propagates that information up
the statements that generate X.  The general idea is to use the
information to simplify instructions (rather than a pure DCE) so
I've simply called it gimple-ssa-backprop.c, to go with
tree-ssa-forwprop.c.

At the moment the only use of the pass is to remove unnecessary
sign operations, so that it's effectively a global version of
fold_strip_sign_ops.  I'm hoping it could be extended in future
to record which bits of an integer are significant.  There are
probably other potential uses too.

A later patch gets rid of fold_strip_sign_ops.

Tested on x86_64-linux-gnu, aarch64-linux-gnu and arm-linux-gnueabi.

gcc/
	* doc/invoke.texi (-fdump-tree-backprop, -fssa-backprop): Document.
	* Makefile.in (OBJS): Add gimple-ssa-backprop.o.
	* common.opt (fssa-backprop): New option.
	* fold-const.h (negate_mathfn_p): Declare.
	* fold-const.c (negate_mathfn_p): Make public.
	* timevar.def (TV_TREE_BACKPROP): New.
	* tree-pass.h (make_pass_backprop): Declare.
	* passes.def (pass_backprop): Add.
	* gimple-ssa-backprop.c: New file.

gcc/testsuite/
	* gcc.dg/tree-ssa/backprop-1.c, gcc.dg/tree-ssa/backprop-2.c,
	gcc.dg/tree-ssa/backprop-3.c, gcc.dg/tree-ssa/backprop-4.c,
	gcc.dg/tree-ssa/backprop-5.c, gcc.dg/tree-ssa/backprop-6.c: New tests.

From-SVN: r229139