[tree-ssa] bnw-simple-branch now usable

[Jason Merrill <jason at redhat dot com>]

I hereby announce that the tree simplifier rewrite and new IR are
functional enough to be useful for C.  The code bootstraps and compares,
and AFAICT the only failures in the C testsuite are from missed
optimizations.  Now would be a good time for interested folks to check it
out and help with porting the current crop of tree optimizers to work on
the new IR.

Unfortunately, I neglected to set a branchpoint tag when I created the
branch, so the only way to get diffs from the branchpoint is via 
cvs update -j.  One approach would be to run

  up "-jbnw-simple-branch:08/28/2002 20:00 GMT" -jbnw-simple-branch

in a checked-out copy of the tree-ssa-20020619-branch.  Or just grab the
patch from

  http://people.redhat.com/jason/bnw-simple.patch.bz2

The merge was pretty smooth, and the resulting compiler works well.  I'd
like to go ahead and push this stuff into the tree-ssa branch and get the
pain out of the way up front, rather than try to deal with optimizers
changing on both branches.  What do y'all think?

Note that this code still uses EXPR_WITH_FILE_LOCATION to provide source
position info: there's one around each statement.  It also still uses
COMPOUND_EXPR to chain statements.  It should be possible to change these
without too much impact on users.  There are various other known future
directions, such as lowering case labels and raising do-loops, but those
can also come later.

I'll be continuing to work on cleaning up the code; currently the C and
generic simplifiers are mixed together in the same file.  I plan to move
the generic code into tree-simple.c.

Enjoy!

Jason