[PATCH, testsuite] add scan-rtl-dump

[Paolo Bonzini]

> ERROR: gcc.dg/fold-abs-3.c: error executing dg-final: no files matched glob patt
> ern "fold-abs-3.c.t[0-9]*.gimple"
That means that I left out not only scanrtl.exp but also scantree.exp.  
I must have typed "testsuite/lib/scanipa.exp" instead of 
"testsuite/lib/scan*.exp".

I now checked that I have no other pending modifications in the tree I 
committed from.  Sorry for screwing up this badly. :-(

Paolo