Fix a nasty latent bug in the legalizer that was triggered by my patch

last night, breaking crafty and twolf.  Make sure that the newly found
legal nodes are themselves not re-legalized until the next iteration.

Also, since this functionality exists now, we can reduce number of legalizer
iterations by depending on this behavior instead of having to misuse 'do
another iteration' to get the same effect.

llvm-svn: 24875