[tree-ssa] A pass to remove a large number of casts in C++ code

[Paolo Bonzini]

> Even though the tree-ssa is in a freeze this
> improves C++ so much I was asked by Diego to submit it.

I'll time a bootstrap with this patch, without ADDRESSOF, and disabling RTL
jump bypassing as well as CSE1; I think I'll be able to submit a patch on
Monday.  Anybody can volunteer to SPEC-test it?

Also, can anybody SPEC-test -O2 vs. "-O2 -fno-gcse"?  It may help to
move -fgcse to -O3 as discussed some time ago.

Paolo