Implement basic block extraction for the miscompilation debugger. This still needs

two things: the FIXME in ExtractBlocks needs to be implemented, and the basic block
extractor itself needs to have enough bugs fixed for this to be more or less
useful.

Until the time that this is generally useful, it is hidden behind the new bugpoint
-enable-block-extraction option.  I hope to get the FIXME done tonight.

Also of note, this patch adds a -extract-bbs option to bugpoint which can be used
to debug the block extractor.  (hint hint Misha :)

llvm-svn: 13471