Mainline bug in RTL fwprop.

[Paolo Bonzini]

> It would be really sweet if we propagated the 'di - 4' into insn 8, then
> recognized that di is now the value of SI 58, and propagated di into the
> compare.  insn 7 would be dead and we'd get the code the PR is looking
> for :-)

Unfortunately there's no hope of that because fwprop doesn't do value
numbering, but I'll try to look at it because it's a wrong code bug.
(It would be great if you had a testcase for mainline...).

Paolo