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.