use register iterators that include self to reduce code duplication in CriticalAntiDepBreaker

This patch addresses 2 FIXME comments that I added to CriticalAntiDepBreaker while fixing PR20020.

Initialize an MCSubRegIterator and an MCRegAliasIterator to include the self reg.

Assuming that works as advertised, there should be functional difference with this patch, just less code.

Also, remove the associated asserts - we're setting those values just before, so the asserts don't do anything meaningful.

Differential Revision: http://reviews.llvm.org/D4566